This depends on what is being calibrated. A four-channel instrument will take longer than a single channel one. A complete sound level meter will take longer than a microphone. Also, a lot of time is spent in the laboratory setting up for a specific instrument type. You can minimise the turnaround time by planning with the laboratory in advance – for example, if several sound level meters of a particular type are to be done, the tests are set up just once, saving time. Also, let the laboratory know the timescales – there’s no point sending in a meter if their input shelf is already full.