Scenario: A solid 45 minutes of intercourse might make you feel like you've earned a badge in boning, but after an hour, it starts to become apparent that you may never pass the finish line. She'll be silently wondering why.
Solution: Don't be afraid to throw in the towel. Honestly, she'd prefer you admit it's not going to happen tonight, rather than have you pump away 'till you're both chafing. She'll understand. Unless it becomes a habit, in which case you might you need to think about what's going on down there. Maybe you're masturbating too much, or with too tight a grip? Tone it down a bit, and you'll both feel the difference.