My last dog walk was an hour dog walk. This means Dudley (my waggy-tailed 'client') was collected, popped in the car, driven about 15 minutes, hopped out and we went for an hour walk (+ 5 minutes because he made a new friend who was coming into the forest as we were leaving

), then 15 minutes back (these 15 minutes include towel-drying his underside and feet (it wasn't raining, but the ground was wet so he'd had up-splash), getting him comfy, and my boots off - we 'chat' while I'm doing this

), and then 10 minutes writing up his daily log for his owners while rubbing his tummy with my free hand amid the dog version of gurgles of pleasure
If I took out two dogs, it would obviously take longer, but the actual walk, from when everyone was inside the gate of the forest (or the first step off the carpark onto the park/woods/etc) to the moment before we got back out of the gate (last step before the carpark) takes about one hour. My route with me walking at my normal speed takes an hour, with dogs that like to sniff every five minutes, it takes up to an hour 15.
There are some dog walking companies that will take your dog out for an hour, and half of that will be in the car. Personally, that would shock me horribly - which is why I don't do it. My clients pay for an hour, they get one hour walk plus travel plus drying off time plus attention and their log filled in. Same reason I never took my own dog (RIP

) on my walks - if your dog walker (that you pick does), ask them about their insurance, from a warning I heard, there are no companies where a dog walker taking their own dog on a professional walk won't invalidate their own insurance, and there are a few where their dog will also invalidate their professional insurance - not good!
As for how many dogs, depends. There are dog walkers that take out 12 at a time (urgh!), and there are those that take out 6 (most insurance covers this, I know mine does), there are those that take out 4 (my personal maximum, and then it'd be four little'uns, not four labradors!), some take out two (Rona above me), and some offer individual walks. Some offer a mix of group or individual. I offer single household walks currently, so either one dog, or dogs from the same home.
All things I'd suggest you discuss with your potential walkers. I know I'd be happy to talk about such things - if people seem worried by the length of time, I offer to use Glympse, so they can see where I am during my walk, so they know exactly how far and how long it takes. Turn it on when I get out of the car, turn it off when I get back in - simple. But realistically, as a pet owner, you want to go with someone who you feel answers you honestly, that your dog likes - fail safes are good though, so don't be afraid to ask. Anyone who is doing what they are saying they are doing won't mind proving it (I know I wouldn't be offended

).