How long to allow inactivity on forum? | Forum Promotion

I’m always on my forums, or forums I’ve joined, looking to post daily, so if I see it’s starting to stagnate, I go and find something related that I can start a thread about, generate the content myself and get people chatting. When it’s your forum, you’ve got to be actively involved in content generation or people will just leave because they won’t think you care enough to be active on your own forum.