Or better yet, use the "on start" event. But then again, if you want it 5 hours after joining a channel, an on join. You can always specify the timer to stay running even if you dissconnect.
Use the -o switch to keep a timer running even if you dissconnect. It all depends on how badly you need to clear the window. You can set the timer to run an alias, use the alias to check $idle so you know if you are active behind the connection before clearing the screen as well.