Well, the speed of a wash can vary greatly as far as what you are wanting to do and more so what the state of the vehicle is in for sure. For just to quickly wash your car to make it look like you don't driving dirt around 15-20 minutes to quickly wash the exterior meaning just the body panels and quick cleaning of wheels shouldn't take you more than 15-20 minutes. However, that's not really detailing a car that's just cleaning. I'd say if your car is about medium in the dirtyness category to detail the exterior (panels, wheels, door jams, windows, trims) should take you around 45-60 minutes, minus any paint correction (2 or 3 step). Sometimes however detailing can not be given any set time due to the fact that you never know how easy or hard a part might be until doing it.
I'm a part time detailer and have a full time job on top of that, so I do want to try and save some time if possible to feel I have some time off in the week. Here is what I do, when accessing the vehicle of what needs to be done, I write it down on a pad and then estimate how long I think the job will take plus my steps on what I'm going to do. It keeps me working towards a goal and not wasting any time that is not needed. Also, consider things like having everything easily accessible to get to. Other than that, things like a foam gun, pressure washer, better techniques and the right products can help cut time quite a bit.