It depends on what all you install. For subs and amps and components, it can eat upto the whole day. morning to eve.
Check the system to your satisfaction before you leave the shop. Its very important. Also, ensure all your stuff is in place. Nothing is missing, keep the remote at a secured place. remove all small wire cut pieces from the car. Ensure that you donot roll down your windows for the next 3 days as you install a sun film. Try and test the system with the installer there and then.
This is all what I have learned from my system.
Last and the most important thing. - You have paid your hard earned money to the acessories guy. You can question him in case the output is not upto your expectations ( realisitic expectations ofcourse) . donot hesistate to question on what is happening during installation.