not sure about newfs,
as for leo's maybe as they get older around 5/6 years of age (they mature around 4 years of age, so I'm told) Bree who is my 4 year old bernese has not grown up and always ready to play lol lol
they may not need as much exercise, but a puppy/adolescence (4 and under) always wants to play and needs adiquite (sp?) exercise, and leash training, to keep their joints muscles and heath heathly and to learn what is acceptable in public.
In cooler weather they go out more often, that's their weather.
Archer (who is 2 now) will go out in whatever weather, as the rain or snow does not penetrate his coat, he does not notice that part of it.
as a pup I had to take him out regardless of the weather to potty train him. and to give him the exercise that he needed.
a bored dog is a very unhappy dog lol lol lol.
socializing with other dogs and people are VERY important and also obedience training on these large dogs.