When I got to that point, my priest told me that it was fine if I only had one. I actually got more grief out of other parishoners about just having a godmother instead of a godfather or both, but Father had no issues with it. I just went ahead and asked her, which I assume is how you do it (of course, I could have just gone off on my own on that one). As for time, I think it varies from godparent to godparent (and godchild to godchild). I'm a handful to deal with in all respects, so mine puts in a lot of time with me. Others may not require as much, so I suppose that's really between you and her.