What do you do to verify timers on centrifuges when the vendor does not provide a specification?

We test the timer against a calibrated stopwatch quarterly. We use a 10% drift (the result must be within +/- 10% of the desired time). For example, if you are timing 5 minutes, your range would be 4.5 to 5.5 minutes. If you are timing 1 minute, the range would be 54 to 66 seconds. I admit that this is completely arbitrary, but it has worked.