As Bob says, for general testing high accuracy isn't all that necessary, and I certainly have a few multi-meters which have never been officially re-calibrated (although I've checked them against my bench meters from time to time).
However.....
Work conforming to British Standard 7671 (IEE Wiring Regulations), requires that certain tests be carried out with instruments which have a current calibration certificate. Those tests include insulation resistance, continuity (of earths & ring conductors), earth-loop impedance, and -- where applicable -- RCD (GFI) trip times/currents.
Calibration for BS7671 would normally be every year, or when anything happens to make one suspect the meter's reliability or accuracy -- overload, dropping from a great height, getting dowsed with water, attack by mutant zombies, etc.
