I've reached the current maximum bonus of +0.00000608 BTC to all free rolls. I don't see the percentages anymore. Will I keep a maximum bonus forever, or can it go down, because the percentages come back?
As delay66rus said it can go back down when you do free rolls. It is a ratio of how much you wager compared to how much you win from free rolls so it will stay on maximum bonus until that ratio falls below a certain level then it drops down one bonus level at a time.