20 on 20 off? That's just enough to be irritating! You'd just start to get your head into the traffic and you'd be relieved. That's no fun.
On the other hand, give me an hour on and an hour off and we're in business!
All my professional life (in the US) we've been talking roughly two hours on and 20-30 minutes off. From an official standpoint, the boss wants you on for two hours and off for 20 minutes.