How long do you have intercourse for?
Have her stop using her hand to finish you and you don't masterbate either. Wait 1 week for sex and see what happens.
Is this a recent development? Masturbation can involve more friction than intercourse, so maybe this extra stimulation is what's doing it for you. In that case, abstaining for a while like Vance2335 says might help. Or if you're taking certain medications, there are some that can delay ejaculation. Another explanation I've heard of is a decrease in sensitivity if you're circumcised - but that's more a gradual thing that men notice over the years. There's also the mental aspect to it - if you're sufficiently "in the mood," it may get you there faster.