Well, my experience is, hunting with a longbow you're gonna get plenty of opportunities for stalks and it really sharpens your skills, just like bowhntr said.
That's why I like bowhunting, and have really enjoyed it the few times I've done it. I usually hunt with a muzzleloader for elk, and once I get inside 100 yards the hunt is pretty much over and I'm packing meat. Not so with a bow!! One of my most memorable hunts was with a longbow and I never came close to getting a shot. I closed to within 40 yards of a small herd early in the hunt, and watched those elk for about an hour before dark, they never knew I was there but I couldn't close the distance for a shot. They were grazing in a small meadow and I was behind a screen of spruce, and it was uncanny how there was always at least once cow looking in my direction. When that cow would shift position, I'd start to take a step but then another one would face me. I must've picked up my foot to take that first step a dozen times, held it motionless, and put it back down without moving. I realized that if I'd been muzzleloader hunting the hunt would've been over within seconds of spotting the elk, and I would've missed that hour and all the other stuff that came after in the next few days, like the massive 7 X 7 that I saw, or the doe and fawn I snuck within spitting distance of, or the Pine Marten that went about his business unaware, and so on.
But to answer the question, I usually gauge the distance at which a stalk is worthwhile by the time it would take me to get over to where the animals are, and how long I think they'll be there. If I'm going to lose sight of 'em on the stalk, I figure anything over 15-20 minutes and they're likely to be somewhere else. That's under good conditions when I'm seeing animals. If I haven't been seeing much I'll go to wherever I see 'em. Two years ago was like that, wasn't seeing any bulls so I glassed off the side of the mountain and saw a whopper way down below around two miles away. There wasn't a good road straight down the mountain so I packed up camp and drove down there, probably a 10-mile drive. I saw the bull again the next morning at around 3/4 mile and hauled ass over there but never did catch up to him. Got to see some nice country in the process though.