50 crunches a day won't be a problem since that isn't really brutalizing them. (not implying it isn't a good workout, just saying it isn't one that'll lay you up for a week).
BUT, you might want to alternate so that you are hitting your abs from different angles with different intensities every other day or so.
*mason twists are fast and relatively easy but effective
*crunches on a ball
*pelvic thrusts while head and shoulders rest on the ball (plus Pooka gets to laugh)
Actually, if you do the Ab Ripper X workout from P90X 3 times a week, you will not need anything else for your abs. seriously.
Part 1
Part 2
You can do the whole thing in less than 15 minutes.