The honest answer to how often you should get your guitar set up is: when it needs it. But that is not very useful if you do not know what ‘needs it’ looks like, so here is a more practical way to think about it.
Once a year is a decent baseline
If you play regularly and you want your guitar to feel its best, once a year is a reasonable starting point. Necks move with changes in temperature and humidity across the seasons. String tension does its thing over time. An annual setup keeps everything in check and means small problems get caught before they become bigger ones.
When you change string gauge
If you switch from 9s to 10s, or 10s to 11s, the guitar needs a setup. Different string gauges put different tension on the neck, and the setup that worked for your old strings will not work for the new ones. This is something a lot of players do not know, and it is why a gauge change that should feel better sometimes feels worse. Always follow a gauge change with a setup.
When something changes
If the action suddenly feels higher than it did, or you are getting buzz you were not getting before, or the tuning is less stable, do not wait. Something has shifted. Get it looked at sooner rather than later.
When you buy a new guitar
New guitars often benefit from a proper setup straight out of the box. Factory setups are generic at best. A setup dialled in for the string gauge you use and the way you play makes a significant difference to how the guitar feels.
When you have been putting up with something
This is the big one. A lot of players gradually adjust to a badly set up guitar and forget what a well set up one feels like. If you have had your guitar for a couple of years and it has never been properly set up, book it in. You will probably be surprised at the difference.
I am based in Sittingbourne, Kent. Electric guitars only. Turnaround is about three days. Get in touch here.