We are currently looking for a rescue dog and put in a preliminary inquiry for a medium sized young dog with relatively few details available at the time. They responded to say that she was not the dog for us (quite rightly once they gave more information) but part of the feedback was that our estimate of how much exercise we could offer daily - 45 mins twice a day almost all off lead - was insufficient for a young, active medium sized dog. This is our minimum - we could offer more at weekends etc. I know very few people who exercise their (often quite big) dogs for this long on a daily basis. Are we grossly underestimating how much excercise a dog needs? We were recommended to thinkg of something smaller / a lot older.
We do 45 minutes in the morning after the school run, where she gets to play with her friends/chase a ball, a quick 15 minutes or so at lunch just so as she can have a wee (we have no garden), unless I'm off work in which case we can be out for a couple of hours. If no long walk at lunch, she gets another quick 15-20 minutes at school pick up time, then 30-45 minutes ball chasing on the park before ds goes to bed.
So 3-4 walks a day and a total of 1 hr 45 mins minimum to 3 hrs maximum. Sometimes more, depending on work, occasionally less if the weathers really shit (she hates the rain!) or if I'm working longer hours.
It's not so much the size of dog but the breed/type that is likely to affect the length and type of walks they need.
Our whippets are medium sized dogs and they are more than happy with a 30-60 minute walk most days as long as they get a good off lead run. They're very much sprinters and though they will happily do longer walks (as long as the weather is nice ) but what they really want to do is sprint around for a bit then go back to bed
It might be worth you looking into whippets if you're interested in sighthounds as they're pretty biddable as hounds go.