TL;DR: It depends. Ok, see ya!
It really does depend, though. Here are a few things to consider.
Before we get too far, though…
First up, and most importantly, is to get to know your current setup. Once you’ve got that starting reference point, you’ll be in a much better position to gauge when it’s time for some setup tweaks.
Some people can feel when things aren’t quite right with their guitar but this isn’t always the most reliable route. For a start, our brains aren’t always as dependable as we’d like and if setup parameters change very gradually over time, it can often be hard to notice until they’re out by miles.
So We Measure
Measuring is better
Measure your current setup. Measure action, and neck relief. If you’ve got a guitar with a trem, measure the height it floats off the body (front and back, if it’s a floating trem).
Now NOTE THESE. Write ‘em down. Most of us have smart phones with note-taking apps so make a note of these measurements so you can find them later.
So, now you have the measurements, at any time you can check back against them to see if any of them have changed.
Why would setup change?
It’s a hunk of wood and wire. Things can change. Some times you might want to think about checking your setup:
Climate can be a big factor — especially if you live somewhere that has bigger swings from one season the the next. Maybe you’ve got humid summers and dry winters. That can take a toll and you’ll want to keep an eye on setup as seasons change. Or as you travel with your guitar.
If you’re playing in a different style of music, you might want your guitar to reflect that change. If you always played with a light touch but now you’re jamming with a bluegrass band, you might need to give your guitar a tweak to accommodate the new style.
Change in String Gauge or Type
You’ll definitely want to check when you’ve gone up or down a gauge of string. Refer back to your ‘before’ measurements to see if you need an adjustment or two.
String type is something that many players forget but there can be fair differences in tension between different types of string, even if they’re the same gauge. Always worth a check after a change.
The Two Times You Should Definitely Setup Your Guitar
OK, this is the bottom line on this ‘how often should I setup’ thing.
1. When you get a new guitar
Or if it’s just new to you. A factory-fresh guitar will usually only have had a cursory once-over.
An older guitar that you’ve just bought — if it’s ever been setup at all — probably won’t be just right for you.
The only advice I’d give is to wait a little while after buying before to set up. Live with the guitar for a bit of time to get to know it and you’ll be in a better position to start tweaking.
2. When it needs it
Yeah, that’s a bit flippant but it’s actually good advice.
Some guitars are more ‘stable’ than others, no matter the environment, or string-gauge, or whatever. Some are rock-solid and others need more attention more often. That’s not necessarily a problem but it’s good to be aware of.
Get to know your guitars. Make some measurements and check back against them. I’d say a good idea is to check twice a year to start with — maybe as the seasons shift. You’ll soon get a handle on how sensitive your guitar is and whether you’ll need some occasional tweaks.