It is fairly rare, I'd go for 2 or 3 real ones out of 10 'orgasms', the rest have likely been fakes to move me on as I do like to try and give, usually after I've taken. A lot of girls move it on without faking it and admit they won't be able or don't want to cum. The fakes are never that convincing though, usually too quick and lack many obvious signs.
Some girls have never orgasmed, they've admitted it. Some can't unless everything is right and they're fully relaxed. Some don't want to while they're working, probably because they have a BF. Others can if they're in the mood with a lot of patience and the right stimulation, usually tongue & fingers.
I get a big kick out of seeing a real orgasm, the flushed chest, neck & face, the involuntary vaginal spasms, the body convulsions, the hyper-sensitivity as they push you away and the warm glowing smile & kiss after it all subsides.