It's 17 days, as the ewe's cycle is 17 days.
So if you raddle him today, he might just catch a ewe or two today and tomorrow who had just finished when he went in, which would tell you he is working.
If he raddles any more after tomorrow, it may mean they weren't cycling or haven't held. If he raddles many or most, chances are he has been firing blanks.
However if he doesn't raddle anything it may mean they all got caught and have held, or that he is not working at all.