Re: sele pump
The pump on my 156 JTS runs between 10 - 12 seconds if the car hasn't been driven for a long periods of of time (i. e overnite). If the car is still warm from being driven - than it depends - from no running time to about 5 seconds.
Smaky, I think that information is not correct. If you haven't driven the car for couple of days and your pump stops priming straight away after only 3 seconds, that pump is not running properly.
I am positive that the pump needs to run for a longer period of time when the car has been inactive in order to pressurize the actuator system correctly.
Although 18 seconds might be a bit too long, as they said, the filter on the end of the high pressure hose might be stuffed or your accummulator is leaking, or the pump itself is getting a bit weak.