overtrained, can't be overtrained?
sory jnn, but if you train abs like me you WILL overtrain them

twice a week tops.
dipends what you want though... I want to improve my ab wheel power... and if you train ab wheel correctly (from standing stance) you cant train abs even twice a week because it's just impossible... you will just wear them out, they won't get better.
First do ab wheel from standing stance 4 sets, then do ab wheel 4 sets from kneeling stance, then do stomac crunches against the wall 4 sets, and then lower abs 4 sets.
each set as many reps as you can.... lol train abs like that every day? Doubt it.