Most "failures" with condoms are a result of sperm leaking around the open end. Pulling out is one way to deal with this. A better way is for her to use a spermicide - jelly, foam, film, suppository. Still, do not linger. The longer you stay in, the more likely some sperm will swim around the end.

Well, after you are done cumming you should pull out. Don't continue doing it after you've cum. But the condom is there to generally keep the sperm out for the most part.

But really, to be on the extra safe side, you could still pull out so that those few little that might leak over the sides, don't, well, leak over the sides while you're inside of her.

I'm babbling. I'll shut up now. lol.