What jj said is correct. The exact reason that it became 7 minutes is because of the rule jj mentioned, getting kicked for inactivity at exactly 5 minutes. When the admins were kind enough to add the warning to that, they kept the 5 minutes that was there before, placed the warning at that point, and then added a 2 minute grace period to allow you to return from afk status. This equals out to 7 minutes (yay I can math! do maths! perform mathification! uhh, mathify... I give up). Anyway, 7 minutes was not an arbitrary number just dreamed up one day, it was merely a result of circumstances, development, and progress happening across the timeline of the server's existence.