Calculation of the latency of a relay
Do you have an oscilloscope handy? A quick way to measure the switching time would be to operate the relay with probes hooked up to the control coil and the switching side of the relay. You would need to hook up some kind of dummy load so you can see it turn on. You would need to be able to monitor both the channels on a single scope, so that you can measure the time delay between the two signals.

Switching time for relays is usually in the single digit millisecond range or lower, so it would be hard to time it yourself.