I've never even tried to fake one. If it's just not working for me, I let her know. There's no sense in endlessly pounding her raw if I'm not even vaguely close to popping. Every time I've begged off was due to loss-of-sensation with a condom. I barely get enough feeling to keep it hard, and never enough to go the course.
Once I moved up from standard to large condoms it got a little better, but there's still not enough friction to Git 'Er Done. I've tried masturbating with a condom on to see if I could find a stroke that WOULD do the deed, but no joy. After 45 minutes or so I get tired and frustrated.