When does one get a new extinguisher or recharge?
It is normal in commercial establishments to have them "formally" inspected/tagged once a year. Generally someone is supposed to make periodic checks that they are still indicating a happy gauge reading more often than that.

If the gauge is not in "the green" then the extinguisher should be recharged immediately (if it is in fact one that can be - many "home" models cannot - those should just be replaced.)

So long as the gauge is in the green, "service" is not required until the tank is due for a hydrostatic test (once every 5 years) which requires emptying it, testing it, and then refilling it. I don't think this really applies to home "disposable" extinguishers, but it does apply to refillable models.

If you happen to have a CO2 type extinguisher, there is not a guage - you just need to weigh it periodically to verify that the charge is still present. The "tare weight" (empty weight) should be stamped on the cylinder, and the CO2 is whatever weight over the tare weight is registered on the scale.