Way over the top, IMO....20-25 mins for smaller joints (under about 10lb) but only 15-20 mins for larger joints. So....it could be done in 5 to 6.5 hours. (Not my guesswork, but from a decent cook-book, btw).
Thanks Ginge, that was sort of my thinking. 10 hours seems way too long. I generally cook gammon for longer than recommended on a very low heat, but it would be ridiculously extend it.