How far in advance do you need to decide test date?

Valid question but it is impossible to have a reliable answer since several things come into play:
1) The test centers are used for various tests such as GMAT, GRE, LSAT, TOEFL, etc, so the availability of space depends on the volume of test takers across GMAT and other tests
2) The closer you are to the admissions deadlines, the harder it is to book appointments for that period. People are procrastinators and many want to wait till the last moment
3) Aspirations and employment status in a certain Metro area (e.g. Detroit vs. Seattle) and a total ratio of applicants vs. test centers.

Not sure if you know but the scheduling system allows you to go online and see appointment availability. That's the best way. You can regularly visit and check availability - that's really the best option. Alternatively, if you want to reschedule, it is a $50 fee unless you are only 2 weeks out - in that case, there is no refund/changes.

P.S. In my experience, you can usually find something a month out in any metro area. Less time is more challenging.