I've been watching my wife play the Leon story for what seems like a good amount of hours over the course of the last week or so, to the point where I forgot that there were other stories and thought this was the whole game.
In terms of length, Leon's campaign is the longest (around 8-10 hours for a first playthrough) followed by Chris's (7-8 hours) then Jake's (5-7 hours) and finally Ada's campaign (4-6 hours). In total, a single playthrough of all 4 campaigns could take over 30 hours or so if you take your time . So yes, this game is pretty massive.
When facing a zombie apocalypse, don't join up with a group. Why? Because you'll end up blowing your own head off due to all the pointless drama caused within.