Viagra?

So my husband just recently went back on his depression medicine and stuff that affects his sex life, so his doctor prescribed him viagra. I think we're using it for the first time tonight in attempts to help put me into labor.

I'm kind of scared, I've been cramping and having back pain for the past two days so I can't handle sex for long periods of time. So does anyone have any experience with it or how long it will make him last?

Please and thank you!