The model is the World Series of Poker TV coverage

[joeydb]

Depends on the casino and the game (1-2 no limit, 2-4 limit...)

It is not typically done as a raw percentage (in the casino) but as a tier system (this much of the flop betting, this much of the turn betting, river, etc) and there is usually a cap. It is nowhere near as high as 20% if I had to guess the ballpark.

Online might due a percentage (rounded to whole numbers) because the counting is that much easier. I haven't done it, so that's also a guess, but it would be feasible without slowing the game down.