I

 usually leave after properly turning over the car and come back to pick it up.  Most service centers offer to contact the owner by SMS or landline when the car is ready for pickup.  

 Some people opt to wait for the car especially if they have nowhere else to go or nothing else to do.  They have lobbies usually equipped with TV sets and cable.  You could only do so much in a lobby if you're to stay there for hours.  Even with the comfort of a sofa and airconditioning, chances are, you'll be bored to death or develop a sore butt for sitting too long.  If you're skipping work and decide to bring your work to the service center, you'll find out that the distractions and waiting area isn't very condusive for working at all.  

This

 is why I commend Ford Libis for providing semi-private cubicles (they're located within the showroom but  separated by partitions).   The cubicles have a spacious desk, comfty chairs, chest-high partitions for a bit more privacy and wifi access.  If you can work online, then bring your laptop with you and you can conduct business while waiting for your car.   They also serve coffee or orange juice upon request free of charge while you wait.  

I'm not bringing my car to have it serviced just to avail of this ammenity but just knowing about this service is elightening for the busy work-oriented car owner who needs to take time out to cater to their car's needs.