These tools are precision instruments, and will typically come certified that the readings they give will have a margin of error of 4% or less. Professionals consider that if this goes over 5% the tool will require calibration, so you don’t get much margin for error! The general rule of thumb is that a calibration should take place roughly after every 3,000 repetitions if you use it regularly, or about every 6 months if the tool only gets occasional use.