Never. Sit ups are dangerous. Instead, substitute other exercise (such as crunches or knee-ups) to work your abs. Working your midsection or core muscle groups should not normally be done more than twice weekly.
Answer
Doing a number of pushups correctly will also improve your abs as a nice side effect!
Every day
From my experience spot reduction doesn't work. During sit-ups won't eliminate fat.
But first i need to know why are doing sit ups, what is your desire result, what is your weight loss goals.
sit-ups will help tone but that's all. And you should never work the same muscles everyday.
3 minutes a day is enough.
after dinner
I'd say about 40 to 50 sit ups.
a fifteen year old should be able to do atleast 110 sit ups in two minutes.
60 minutes.
1
Sit ups do not burn stomach fat. Sit ups build and tone stomach muscles.
On the ab rocket you should do the amount of sit ups you need to until you reach you reach your limit ( your stomach starts cramping/ hurting)
Twice a day, do sit-ups for three minutes. Overtime, you'll get better. I can do about 65 sit-ups per minute.
5
sit-ups raise your heart beat more
sit ups measure your stomach strength.
at the last smackdown vs raw sit up/ push up contest john cena, (raw) did 112 push ups and 116 sit ups. he came second to batista (smackdown) who did 117 push ups and 119 sit ups