On long rides, I ride OOS for about 30" every 15' by the clock. That gives a little time to get blood back into those butt tissues and varies the terrain in which I do OOS. I'm 5'6" and weigh 145-150. I find I do best if I ride OOS in the drops, no weight on hands, maybe pulling up slightly, backs of thighs brushing the saddle's nose. Something about that position seems to make it somewhat less work. I agree, watch your power meter. Way easy to go too hard.