most shops ive hung out in are a time/effort basis... you get a ride in that needs 3-4 days worth of work done, you get started on it, if a car pulls in for a sub hour install, you knock it out, get paid and go back to the big project.

time is money, so you tell the customer with the larger project that its going to take longer then you know it will so as to allow for the little jobs that will gain you quick money...

edit: then again it also depends on how big your shop is, how many people are working for you, and what actually is going on. most shops ive been at have been 5-10 car bays and 20+ people working there. shit is always going on!