In my experience perpetual check results in a third repetition more often than not. If I get one I just click "claim draw" with every move and let the system tell me when it's valid, so I don't really know the answer to your question.
Originally posted by Ian47 When can a perpetual check end a game? Three or thirty-three?
I don't know where you get the number 33 from.
This is from FAQ and I think is a universal rule of the game
"No piece capture or pawn movement has occurred for over 50 consecutive moves (100 half moves). Therefore the game has not progressed and a draw may be claimed by either player during their next move."