I typically walk Jackson once a day for between 45 minutes-2 hours depending on how much time I have. It's generally an hour walk per day, a few days may be skipped here and there but that's our general schedule. I live on 3 acres of land, with no sidewalks/neighborhoods so I have to drive 5 minutes up the road to a neighborhood to walk him. We do not have a fenced in yard but he's on a 20 foot flexi lead out back to potty during the mornings and nights, etc. I also take him to a dog park around 3x a week.
I do think it's important for them to go on walks, even if you have a fenced in yard, because it fulfills their primal instinct to walk as a "pack." I think it's good for them to get new smells, sights and experiences as well. When we visit over at my dad's house, he has a fenced in yard w/ their two dogs but we still try to take them all for a walk around the block still. It drains more of their energy, believe it or not, even if they've been running around out back all day. Plus, the walks benefit me as well, getting more exercise.
Jackson LOVES to walk. He does tire quickly in the hotter weather but he could go on for a while. We walked for 2 hours the other day and he did fine. Was pooped when we got home though - which is the purpose!
