>What you really want to shoot for is at least an hour or two.
you pumping away at her for an hour or two without climaxing won't make her feel insecure and unsexy at all...
don't worry about it so much. just because you come, doesn't mean the session's over. make out a bit, go down on her, and you'll be back in action before you know it. if she's worth your time at all, she'll feel flattered that she affects you so strongly. if you can't trust her to not judge you negatively for something as irrelevant as this, you've got bigger problems with your relationship than being premature.