I would first suggest you change your mind set of " you're on them for to long". The actual act of intercourse is a small fraction of the whole experience. Relax, learn how to explore her body and find those spots that bring excitement and pleasure. If you go about it in that manner you will have a very happy partner in the end. Just a side note, I have had experiences when intercourse never occurred and both parties were completely satisfied. Think beyond your dick is basically what I'm saying.
Have fun and prosper young man.
Excellent advice!