I personally do the majority of my backpacking solo; in fact it's a big part of the reason I backpack (to be alone) and also why I get out so often (don't have to coordinate with others on trip plans, etc.).
How far depends on where I am. Living in the southeast, it wouldn't bother me to go out for a couple weeks or more. It's pretty much impossible to be more than a day's hike away from a road, and chances are you'll see someone in the backcountry if you need help. In fact, I'd say in most places in the east you'd have a hard time getting out of rescue whistle range, except maybe in winter; there's probably someone close enough to hear it, whether you know it or not!
Other places I've hiked aren't quite so populated; places like Dark Canyon, or the eastern side of the Wind Rivers, etc. I usually limit my trips in places like that to 5 days. I think I can survive 4-5 days if I have an accident, and after that, I know someone is looking for me! With a PLB, I'd be willing to travel further, and for a longer time, in desolate places like this.
Anyone know where I can get a good deal on a PLB? ;)