Yeah, 6-10 hours is a good estimate of time. (We tend to put slices in our bristlenose pleco tanks maybe one night a week, before we go to bed. There hardly ever is any left in the morning.
If they don't eat it all try putting a smaller piece in next time. I would keep putting a smaller piece until it gets eatten. When they do consume it all put a larger piece in until they stop eatting it all again. That would show you how much you need to put in the tank for them.
We don't have ottos, but our plecos eat every bit of it except maybe 1/2 the outter bits. (Some people call them peels, some call them skin, some call them flesh... whatever)
Good luck.
