I'm no expert here.... but I'd venture up to four months is no big deal, even five if were talking about a move to a new city. It's easy to explain: "I left work and took some time to travel, then returned and planned my move to X". You can also play with the dates a bit ... for instance, if school starts in Sept, but you are talking some prep course in August, just list from August. No one is going to call you on it.
I personally plan on taking 3 - June, July and August. Really, its more like 4 months because I have a 3 week vacation planned in May. The plan is to take my vacation, come back, quit, take it easy for 2 weeks, and go back on vacation.
Largely I'd argue - it depends how your resume looks too. I mean, if you've had one job, a four month gap before school is nothing. If you've had six jobs in four years - then its probably less of a good idea (no staying power).
The only other thing I'm considering is going to my firm and asking them if they'd be interested in part-timing me through June. I need to find out some important details first, but I'd be happy working 3 days a week through June.
Anyone want to just give me $30,000 so I can quit now?