How to translate "phase" into "delay" Suppose that a system induces a phase of -90 degree, how do I practically measure the delay in the signal coming out of the system? I think time lag would be a more useful measure than phase. 

Clearly these have different units, what is the conversion factor that tells me how much time lag is induced due to the system?
The phase delay to time delay conversion depends on the frequency of the signal for which that phase delay is associated.  

Note that unless the phase delay is given as correctly "un-wrapped", the absolute time delay could be ambiguous by integer multiples of the period of the frequency at which the phase delay was specified or measured.