It takes about an hour or more per track I hear. Mastering is a pretty quick thing, it doesn't really help kneading the track around too much, unless the engineer is really inexperienced and doesn't know what he's supposed to be doing. A professional ME knows what his equipment does and what it can do to that particular track even before he touches anything.
On optimum mixdown the mastering affects the sound very little. Nowadays the most audible effect of mastering is turning the track as loud as possible without pumping.