having worked at performance/dyno shop during college, it really is a give and take situation. As said if it takes diagnosing, knock that out first and wait for approval to move on repairing. Big jobs (builds/race prep/caging) are your main task (on going) and small side jobs will roll in and get knocked out quickly just to keep things rolling and to keep from losing a customer.
Jobs that require lots of work, fabrication, custom work, etc...generally is a 1 week+ project and the customer is told that ahead of time. That is assuming the parts are on hand before the car arrives.
I always preffered customers to hold cars until parts arrived and then drop off for work. Keeps more open space and no down time on lifts.