Consider a simple queue model like the one described in http://en.wikipedia.org/wiki/M/M/1_model. The article states what the expected waiting time is before a request enters the queue.
Assuming that the actual queue length is unknown, does the expected value of "time left to wait" for a given request change over time while the request is in the queue?
Clarification: is it true that the expected time until a request is served stays constant, regardless of how long it has already spent in the queue?