There are lots of different schools of thought on this issue, but a good rule of thumb is to have your unit serviced twice a year - one before the summer season, and one after. Here’s why:
Before Summer - It’s good to have service before the weather gets warm so you can catch any little issues and fix them then rather than let them turn into major problems just when you need your AC most.
After Summer - Your unit will by far work harder during the summer months than any other time of year. It’s good to check on it after that grueling season to address any wear and tear issues that have come up.
A regular maintenance plan is vital not only to keep your system running well, but also to make sure it lasts every bit of its expected lifespan.