2-3inches is the standard. When I had do my own mounting (even though I pleaded with my art teacher that it sucked
), we'd do 2 inches on the top and sides and 3 inches at the bottom. This is some sort of standard, it's suppose to "weigh" the picture and I guess it makes anything abstract more obvious as to which way up it is. You'd probably scale that up or down depending on the size of the work. Sometimes 2-3 inches would look silly on a smaller or larger piece (I'd say 2-3 is about right for 8x10 size so probably the same for 8x12, that's only a difference of one inch on either side).
If you're having them done, though, aren't you talking to a competent framer who can advise you as to what will look best? I quite like framers because it's another person to bounce presentation ideas off, if nothing else and they're quite good at picking the best off-white to show off work etc.