GSoC should be treated as a full-time job, that's how much time you're expected to dedicate to it. Having bachelor's degree exams in June would be a serious problem. You'll need to discuss with the potential mentors on how to make up for that time.
My bachelor's diploma is almost done and I will have enough time for GSoC work.
Other than that, the schedule seems fairly relaxed. In fact, this project seems a bit too small for a GSoC project. I'd suggest coming up with some additional GiST-related work that you could do, in addition to the microvacuum thing. Otherwise I think there's a risk that you finish the patch in May, and have nothing to do for the rest of the summer.
I want to take additional work-item for gsoc 2015.
I don't known, which item of todo is completed, but I compose list of items:
1) add support for microvacuum for GIN index in common with Anastasiya Lubennikova (she will be realize function amgettuple in GIN), if it's a possible feature.