Indefinite - providing you respond to any message Nabble sends warning you of a planned deletion.
I think it's something over three months of complete inactivity - i.e. no posts and no views that would generate a warning from Nabble that that part of your forum is scheduled for deletion. I think you normally have a week to respond - just a matter of clicking a link to keep it alive.
I'm sure a member of the Nabble team will be along soon to correct me!
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.