Yes, i guess there are differences, and I am sure there are some that have an attention span that long, I guess I am from a later generation
Can't really say about the grateful dead, I couldn't name one of their songs and I have seen several groups that have done 3 hours+ - Jethro tull springs to mind as quite long, I guess hawkwind might have been too, but I doubt I was in a state of mind able to perceive time during their gigs (doubt they were either).
Don't get me wrong, we have people who watch our entire sets, in fact at one gig a few weeks back there was a woman who started dancing at our first song and carried on until we finished. I just don't understand it!