A dragon’s breath weapon takes an average of 18 seconds to recharge.
Most dragon breath weapons contain the (Recharge 5-6) notation, which means:
For example, “Recharge 5–6” means a monster can use the special ability once. Then, at the start of the monster’s turn, it regains the use of that ability if it rolls a 5 or 6 on a d6.
So each round, there is a 1 in 3 chance the weapon recharges, and so with six second rounds, you can expect an average recharge time of 18 seconds. Thus, it would be highly unusual for a dragon to not recharge its breath in 60 seconds (about a 1.7% chance of not recharging 10 rounds in a row).
The DM decides how the dragon behaves.
what’s to prevent a dragon from swooping down from a great height, breathing fire on a party, then ascending into the sky again (beyond the reach of most ranged weapons) waiting 2-3 rounds to recharge, then swooping down on the party again?
And the answer to this is, well, nothing, except that the DM doesn’t want the dragon to do this. You have identified a tactical advantage that dragons have when battling in the open air. The range of an adult dragon’s breath weapon keeps them out of range of melee opportunity attacks, so readied ranged attacks and spells are the only option.
But the DM needs to consider “how can I make this encounter fun” before considering “how can I beat the players”. Unless “DM vs. Players” is agreed to prior to starting play, D&D is by default a cooperative game, not “DM vs. Players”:
The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game. That said, your goal isn’t to slaughter the adventurers but to create a campaign world that revolves around their actions and decisions, and to keep your players coming back for more! If you’re lucky, the events of your campaign will echo in the memories of your players long after the final game session is concluded.
The success of a D&D game hinges on your ability to entertain the other players at the game table. Whereas their role is to create characters (the protagonists of the campaign), breathe life into them, and help steer the campaign through their characters’ actions, your role is to keep the players (and yourself) interested and immersed in the world you’ve created, and to let their characters do awesome things.
Knowing what your players enjoy most about the D&D game helps you create and run adventures that they will enjoy and remember. Once you know which of the following activities each player in your group enjoys the most, you can tailor adventures that satisfy your players’ preferences as much as possible, thus keeping them engaged.
Surprising the players with an unwinnable encounter is usually a bad idea unless you’ve got some other tricks up your sleeve to keep the players engaged and interested. Getting hit-and-run over and over by a dragon isn’t going to be particularly fun, but a head-to-head struggle in the dirt might be, and will still be a difficult encounter.
I use “open air dragons” for show and tell, not the actual encounter.
What I gave above is the “metagame” reason for dragons not to engage in “open air” combat with the players. But I have been able to rationalize it within the narrative of my own campaigns just fine. Unless the party has done something to severely upset a particular dragon, the dragon probably doesn’t have any reason to just engage the party outside of their lair. Depending on the lair features and your use of lair actions, a fight with a dragon can be just as difficult, if not more difficult than the situation you present. But there is one key difference: the difficulty provided by the lair and lair actions is typically going to be far more interesting than hit-and-run open air tactics. There is just a lot more going on in the lair that won’t be quite as frustrating as getting kited in a field.
And since you’re going for something true to a dragon’s preferred tactics, they’re going to prefer fighting in their lair anyway:
Dangerous Lairs. A dragon’s lair serves as the seat of its power and a vault for its treasure. With its innate toughness and tolerance for severe environmental effects, a dragon selects or builds a lair not for shelter but for defense, favoring multiple entrances and exits, and security for its hoard.
Most chromatic dragon lairs are hidden in dangerous and remote locations to prevent all but the most audacious mortals from reaching them. A black dragon might lair in the heart of a vast swamp, while a red dragon might claim the caldera of an active volcano. In addition to the natural defenses of their lairs, powerful chromatic dragons use magical guardians, traps, and subservient creatures to protect their treasures.
Which brings to what I mean by “show and tell”. When I use a dragon in my stories, if there is going to be a fight, it’s going to occur in the lair. When the dragon is seen flying around in the sky, it’s sending a message to the party. “Hey guys, there’s a dragon here, so watch out for dragon related plot devices”. And then when it comes time to personally introduce our dragon, I will use an open air encounter to give the party an idea of what they are up against, without intending to carry on the fight. So dragon campaign arcs, for me, usually have three actual appearances of the dragon:
1. Observe that a dragon exists
The party sees a notice on the bulletin board at the local tavern, “Adventurers Wanted Up North”, asking them to meet with the governor of Up-North-Land. While travelling, once the party is near their destination, they will observe the dragon. “As you break the crest of the last low mountain, you peer down into a snowy valley. Across the valley, you catch a glimpse of something catching the late afternoon sun – something you have heard stories about, but have never seen with your own eyes. The glistening outline of a silver or white dragon circles in the sky above the far mountains across the valley.”
2. Taste the power of the dragon
You never want your party to try to take on the dragon to early. You don’t want them to force you into a situation where you have to kill them or make the dragon do dumb stuff. This is where this second appearance of the dragon comes in handy. Find an opportunity in your story to put the dragons true power on display, before the party is committed to a fight with them. In this campaign with the white dragon, my players were tasked with stealing a powerful relic from an ancient temple that was guarded by the white dragon. But they didn’t know it was guarded by the white dragon. They had the bright idea to walk up to the front door and knock, so on the way, I had them meet the white dragon. I had the dragon swoop down while they were navigating across the tundra to the temple and blast them with Cold Breath, then fly off in the direction of the temple. They got the memo. After regrouping and doing some more research, they learned of a secret underground entrance to the temple and were able to get in and out with alerting the dragon.
3. Actually fight the dragon.
Eventually, this campaign came down to a showdown with the dragon in the dragon’s lair. At this point, they had seen the dragon a few other times, but the one brief encounter where they got blasted with cold breath was enough to be sure they were prepared.