6-8 hours is ideal but it is still risky, if it is a main account and hasn't been banned previously they might hit you with a 14 day ban, if so then you should probably stop.
It is usually recommended to use a vpn because jagex might have your ip flagged, and like others have said it doesn't matter how short of an amount you bot for you can still get banned, I once had an account banned that I botted 30 minutes on