Here's how I plan on doing it:
First, each attempt takes a certain amount of time. For escaping from manacles I'd set that at 30 minutes.
Second, taking 20 takes 20 times a long. Not necessarily true, from a statistics point of view, but I'm comfortable with that.
So, under those rules, escaping from manacles takes 10 hours. If the PC has 10 hours (for example the PC was manacled and then put somewhere while the bad guys went about their evil plan) then that's o.k.
If I think the skill check would take an action, then it takes 2 minutes (i.e., 20 rounds).
If the PC doesn't have that kind of time, then they would have the option to re-roll until they ran out of time. Or they could try something else. This seems the way that is fairest to the players, while not seeming arbitrary.