I'm 21, 5', 123 lbs woman. i exercise regularly 30-40 minutes (spot running, sit ups, stretch). so how will i know these are enough for me? other advices

Not sure of question: I am not sure what you are asking. It sounds like you are doing some regular exercise which is good. You have some cardiovascular, and stretching. Sit-ups are likely not doing what you think, by the way. I believe the best thing for you is to set goals and discuss with a trainer or someone who understands exercise and design a plan for you. Also watch diet, stress reduction, habits for good health.