Well, it depends how close to the exam it is. If it's pretty close and it's weekend/holidays, it looks like this:
10-10:30am - Subject 1
10:45-11:15 - S2
11:30-12:00 - S3
2:30-3pm - S4
3:15-3:45 - S5
4-4:30pm - S6
With the breaks (if you do something unrelated) it allows time for the information to sink in, so your brain doesn't get 'overloaded'.
You don't get loads done in half an hour, but the thing is, it makes no difference, because you'll still have the same amount of time to revise as if you were doing it in 1 hour slots, it's just broken down more. It means you're more 'sharp'; you're going to work much harder for two separate half-hour slots, and learn more, than a 1 hr one, because you'll end up slacking off or your brain going a bit 'fuzzy' by the end of the hour, if you see what I mean.
It's also important to go back over things: