Okay, look. Let's use that complete Venus map as a base to try and nail this thing down. If there are 5 playable locations (Earth, Moon, Mars, Venus, Reef), each of which having 7 story missions, 2 strikes, and a raid (again, JUST going off that Venus map here. This is likely to be wildly optimistic), we're looking at 35 story missions, 12 strikes, and 6 raids. Let's continue to be generous and put the times at 20 minutes for each story mission, 45 for each strike, and 90 for each raid. That totals to juuuuuust under 30 hours, at the absolute most; if the other zones are less populated than Venus, or one of them is indeed just another social area, it's going to be considerably less.
That's pretty respectable for a shooter. Unfortunately, Destiny has been billed as also being an RPG, and a 30 hour RPG just barely clears the threshold of miserable disappointment. Explore mode can pad the numbers a little, but unless the release explore maps are considerably more fleshed out (more secrets, more interesting missions, more events), Destiny is looking like it's going to feature a loooooooot of grinding at release. I think it's fair for people to be apprehensive about that, especially in light of the story they presented in the beta (still hope there's more to that at launch.)