How often do you have sex?

I've been with my boyfriend for a year and a half. We usually have sex at least twice a week, but recently it's been over two weeks since we've done anything to each other. I've been busy with final exams and he's been working more. I've noticed he's less patient with me when we go through a "dry spell" like this. I don't want to have him feel like he doesn't have enough sex. How often do you have sex with your boyfriend to keep him content and happy?