It depends on alot of things I guess. My most recent horse was around 1 1/2-2 hours away and I was sooo excited to have found one (well, THE one) so close! The horse my sister bought (who is technically my most recent I guess since he's been dumped on me) we spent about 15 hours driving, but that included delivering a mule I'd sold (it was on the way though), and we looked at two horses not one (who were not exactly close together).
Right now I wouldn't have the money to drive to far, but then again I don't have the money for another horse either (not that I want or need one). But if I did need another, and I had some money saved up, I'd go as far as I could afford to for the right horse, but I'd have to be pretty sure it was the one, before commiting to going that far. What we did when we looked at the last horse (sister's, now mine) is we found several in the area that we could also look at if the one we went to see wasn't the one (which was the case, the first one was a definate no).
I've seen people not look at a horse because it's soooo far away, and it's an hour away, but then I'm guessing they have a much broader idea of what they are looking for then I do. And/or they may look at 10+ horses before finding what they are looking for, I rarely look at more then one when horse shopping, and if I had to look at that many it may get to expensive and that can affect how far someone would be willing to go and look.