Well I can give you some personal experience and let you decide on your own. When I first started doing situps, I was "doing" 60.... or so I thought. Honestly what happend was I was doing about 30-35 and then cheating/fighting/resting from 35-60. I would only do 2 sets of these a day, and that was my abs workout. 120, with prob half in correct.
So I dropped down to reps of 30, and do them correctly without cheating. I also do A LOT more sets, but spread out... by the end of the day I do 300 situps, so 10 sets of 30. This is arms behind head to, if you have your arms on your chest, or sides or whatever they are A LOT easier. If you have your arms behind your head you use your ab muscules more, and thats the whole point.
Anyway, back when I was doing the 60x2 a day, I could barely do 25 seconds of flutter kicks, and dead cockroaches was pure hell and couldn't last long at all.
With my new method, which I do 4-5 times a week, I am flutter kicking like a champ, dead cockroaches are easy, and even frog kicks aren't much of a problem any more (if you don't know what any of these exercises are just ask).
My opinion is 6 or 7 days a week is too much, but 4-5 (which is still a lot, can you imagine doing something like... squats 4-5 times a week? no way, lol) and come out ok! At least I am
