There are full-time jobs out there with 40-hour weeks or less, but they're not very common and obviously they don't pay as well. There are also plenty of part-time jobs, but those obviously won't pay enough to pay off student loans, etc. If working limited hours is a huge priority to you, then you may want to look at another field.
I'm a small animal vet, and I've averaged about 40-50 hours/wk in a total of four jobs over the last seven years. The most common SA schedule in my area is four full days per week (usually a 10ish-hr day... most clinics are open 9.5-10 hours, and you may get to take a lunch break but you also will likely have to stay past closing most days) and a half-day every other weekend. Also, you'll factor in an additional 3-5 hours per week from home for keeping up to date on veterinary journals, researching tough cases, etc. I hear that some people have time within their workday for those sorts of things, but I've never worked at a slow enough clinic for that to be the case - I've always been with patients from the time I walk in the door until the time I leave.
I currently work for a corporate practice and I'm scheduled 9a-7p three weekdays per week and 9a-5p on Saturday. I REALLY love my schedule right now, but keep in mind that many days I don't get lunch and most nights I'm there at least an hour after closing.... so it still works out to about 40-45 hrs/wk plus the additional time from home that I mentioned. Also, there's no flexibility in my schedule.... days off have to be requested at least six weeks in advance and are not always granted, calling in sick is really frowned upon unless you are seriously ill (that's the case at all vet clinics - I've called in twice in the seven years that I've been in practice), and coming in late / leaving early is absolutely not an option so I miss out on a lot of evening events because I'm usually stuck at work until at least 8pm or so on days that I work. Also, I was just recently allowed to go back down to this cushy schedule after three months of being made to work a lot of 5-day (50-55 hr) weeks. I get paid extra when I take on those extra days, but it really burns me out.... luckily, after three months of begging/pleading and finally just refusing to work any more long weeks (which was a scary move, because I half-expected them to just fire me), I got back down to this schedule. For now, anyway.
I'd love a 9-5 job, but it's really not an option for a vet unless you own your practice (and the only way to get to that point is to work a lot of really LONG weeks, so again not really a path for someone who wants a lot of work-life balance). If a 40-hr week is a huge motivating factor for you, I'd definitely start looking into other career options.