It varies, but usually about 2x/week in the gym (half weighted half unweighted exercises) and maybe once a week at home (not weighted) on my Swiss ball. In the gym I'll do sets of abs in between sets for other bodyparts, maybe for 9 sets total in the range of 15-20 reps per set. I don't really count, but I think that's about right. I really concentrate on squeezing hard on each rep.