Well, I have an FAA fixed-wing license, and am about to take the checkride for my helicopter add-on. I have 40 helicopter hours, and you strictly need 30 for the add-on.
The main reason for going over was not planning time very efficiently, but I wasn't too pushed about getting it right at 30 anyway - hours are hours, and I love to fly.
Generally speaking, I think people tend to complete the rating (either initial or add-on) with about 10 hours over what's required. Some take less, some more.
Are you planning on taking an intensive course, or the old 'once or twice a week' approach?
Peter