OSHA mandates air sample testing every 6 months if the compressor is used in commercial diving operations. This includes businesses with employees that provide "scuba instruction and guide services" (so most all dive shops). PADI used to require quarterly testing for their "5-star/golden banana" shops, but yielded to the dive shop owners who complained that was too often.
That said, regular testing by itself is no guarentee that everything is A-Ok. It's just a capture at a specific point in time. The very next day, the compressor could overheat, start burning oil, and jack the CO through the roof. Amongst any number of other possible failures.
I have a small compressor that gets about 50 hours of use each year. I do an OCA test every 6 months. I do regular maintenance (oil and filters) at about 80% of when it should be done. I have inline moisture and CO monitoring, and keep great maintenance records. I don't fill commercially, but the buddies I fill for are welcome to inspect any or all of my fill station at any time. Questions are encouraged.
You should expect at least the same from your local dive shop. If they aren't at least that rigorous, toss the whole silly "life support" argument back in their face.
Filters are $50-75, air tests are $75-100, and compressor oil is about $100 a gallon. All said, maintenance makes up a couple dollars of your fill cost. Make sure they're actually doing it.
Remember: The shops are going to harp on you for regular maintenance, including: vis inspections, hydros, reg service and possibly oxygen cleaning. What's sauce for the goose is sauce for the gander.