Depends on the service. Varies hugely and also depends on the timing, type of car etc. It isn't just about oil, filter and sparks. There are visual checks on suspension components, exhaust, changing diff and transfer box oils etc.
For example, I just had a 48 month service on my GT-R and it was a 3-4 hour job checking through a huge list of stuff. Also needed wheel alignment and resetting clutch points, idle learning etc.
So depends on:
1. Car
2. Age of car
3. Where you take it
4. If there are other bits that need doing like brake discs/pads etc.
I find good independents are far better than dealers.