How many times can I have a sex in a month?

My girlfriend and I have been dating for one and a half years and are in a long distance relationship (I visit her once a month). When we first starting dating (it wasn't long distance then) she was crazy in bed and really open about sex. But recently she has been wanting to talk less and less about sex. She doesn't say anything if I even mention something that is sexually related. I asked her why, and she said that she doesn't feel like thinking about that kind of stuff and that she feels dirty for thinking about it. She doesn't seem to have ANY sort of sexual attraction to me when just a few months ago she couldn't keep her hands off of me. I understand that she is going to college and is very stressed out, but can stress completely shut off someone's sex drive? How can I talk to her without offending her or making her think that sex is all that I am interested in.

I’m a 14 year old boy. There is this girl that really likes me and I feel the same way about her. We want to have sex, but we’re both uneasy about it. What should we do?

Can you still get pregnant if you only have sex for 30 seconds?