How much sex is normal

okay, i get bitched at by my gf if i dont give it too her... we have relationship problems and the sex is not that good....oh and she bitches about everything all the time.. literally (shes on psych meds). my question is... how much is average? im 22, we do it 2-3 time a week. were both stressed from being senoirs in college (full time) and both work 2- 12 hours shifts a week on different days.