actually i think abs can be worked quite more often than other bodyparts, but it all dependy on the volume ithink. so doiong every day just some 3 sets of crunches would be ok. but i wouldn´t recommend working abs with a higer volume and higer intensity(6-8sets all to failure) daily. in that case 2-3times a week would be really enough. if you really train your abs hard than also once a week would be sufficient i think. well, you see concerning abs you got a lot of option. i do them twice a week in the gym and sometimes a third time at home, just some crunches and leg raises.