Consider making the effort to get out a skill challenge -- it could take a variety of different skills (acrobatics to wriggle free, athletics to burst free, thievery to manage to untie the knots). Then you can decide that a "round" in the skill challenge requires one minute of game time.
Another interesting wrinkle: rather than ending the challenge at 3 failures (I mean, would the PC really stop trying?), you can allow an unlimited number of failures, but make the DC +1 higher every time the PC fails a check (as the knots get tighter and tighter, etc.)
Set the DC at an appropriate number -- probably at least the hard DC in the (revised)
DMG.
-rg
And yet another word from our sponsors
