Personally, I would only offer a BJ or a HJ for 15 minutes.... maybe a titty fuck, but certainly not full-on penetration. It really is all up to you, though, and what you're most comfortable with. I've noticed when you're new, it is really easy to be exploited. So yeah, don't be afraid to up your prices. If they need you that badly, they can pay for you. And there will always be clients willing to pay, so you don't have to worry about that lol.
I pretty much only offer 15 minutes of service, because I like to be in and out. The clients do tend to push for more time, sometimes, but if you're firm, they can't really say much. They knew how much time they were buying at the beginning...