What happens with the time that was spent on the undid move? Is it lost time, or added back to the clock? If the latter, how would you avoid abusing the undo to

What happens with the time that was spent on the undid move? Is it lost time, or added back to the clock? If the latter, how would you avoid abusing the undo to gain time? (E.g. think 5 minutes on a move, undo it, and then do it again, but this time in 2 seconds)