Looking up is great - I try to do it often - but on some of the more rugged trails in the Rockies you'll end up face-down kissing those rocks in the trail if you're not careful.

When I'm hiking with friends, we're either a few feet away from each other jabbering up a storm, or far more than 20 steps apart hiking in silence, enjoying the wilderness. If you cannot reaquire line of sight to your hiking buddy after stopping for 30 seconds, you've gotten too far apart though. Generally, I'd say we can be within "touching distance" in 60 seconds or less when we're spread out into our silence zones.