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!
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.
1
60 minutes.
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)
Sit ups do not burn stomach fat. Sit ups build and tone stomach muscles.
5
Twice a day, do sit-ups for three minutes. Overtime, you'll get better. I can do about 65 sit-ups per minute.
sit-ups raise your heart beat more
sit ups measure your stomach strength.
5X20