Why 6 months? More like 1 month. If you plan to submit your school list in the summer, April or May is the earliest you should buy. That gives you plenty of time to go through it and pick schools that you might fit with. I believe the new MSAR is published every year in April, so that would give you the most current version.
^This advice is just for the online version, which expires 12 months after purchasing. I bought mine last May and it just expired, I pretty much used it up until the day it expired so I'm glad I waited until May to buy it.