I'd rather just drop the arbitrary stopping point. If a player asks me why he can't try again and I can't give him a good answer, then of course he gets to try again. Maybe I just find verisimilitude to be more important than others though.
I understand where this view comes form, but (IME) it means that some skill challenges stop being interesting -- it becomes a "take 20" rule, and the DC is either set at a level where it is possible to overcome, or it is not.
One variant, inspired by the FATE system, would solve the problem in my view:
assuming the DC is within reach of the roll, the number by which the roll is missed is the number of minutes (not rounds) that are spent working on the lock. That's not a perfect solution (and ideally it would require an initial statement from the player along the lines of "I try the lock, and if I don't get it right away, I'll work at it for five minutes" (whereby the player is allowing herself to miss the roll by five). But eventually, human beings give up when they are unsuccessful -- this should be reflected in any skill roll, and is why a single roll is (for me) the cleanest expression of verisimilitude.