Have you discussed this concern with your girlfriend? I'm thinking that if there is a lot of foreplay and the lady is eager for penetration, 5 minutes of thrusting should be plenty for her to reach orgasm.
If she isn't, maybe the two of you are not spending enough time on erotic moves before penetration. (And conversely, if she *is* reaching orgasm, it's also likely that 15 minutes of thrusting is not more desirable to her than 5 minutes.)
If you know your girlfriend doesn't particularly care how long the thrusting part of sex goes, that should help you stop focusing on 5 minutes versus 15. It could mean sometimes you will come fast and other times more slowly, but it will be no big deal either way. Basically, if you know she's getting what she wants from the sex, it should be easy to let go of worries about how long you thrust.
If your girlfriend says she is not reaching orgasm, try looking up creative foreplay methods and see what those do for the two of you. (Someone who is panting for you to penetrate her because the foreplay got her so excited, should reach orgasm pretty fast.)
No matter what else, talking about it is a good thing. Don't start having sex and talk about it then, talk about it after dinner or some other relaxed time.
Good luck!