How far in advance do I need to apply for a retirement visa to move to Thailand in September?

There is no such things as retirement evisa

Have you been on to Thailand longer than 2 or 3 weeks?

Before moving you should try to stay a long period (ex. 60 days touristvisa + 30 extension at least) in Thailand, since the dreamscenario based on a few weeks holiday, may be far from what everyday life is.

Depends on which retirement visa you'll be applying for.

A non-O visa is valid for entering Thailand for 90 days from the day it's issued. So you can apply any point between 90 days before your trip up until about a month before to be safe.

A non-OA is valid for a year and if you want to maximize your time with it, you'd want to apply as close as possible to your trip while still giving yourself enough time for the visa to be issued before you leave. But the validity of the visa starts ticking the day it's issued.

Check the Thai Embassy website in your country.