Actually, there is one scenario where it is too late for maintenance—your air conditioner has completely broken down and the compressor is shot. In this case, you’ll likely need to replace the compressor—which at this point we’d recommend an entire system replacement.
This is a lot less likely to happen in a newer system, or a system that has had maintenance done on it once a year. That’s the key, that your AC maintenance be done on a regular basis, no matter what time of the year it is. Sure, it’s ideal to have it done in the spring before temperatures get too high, but this is definitely a case where frequency matters more than timing.