Lets do a quick pretend melee check, 25 characters, if it takes like 15 minutes to beat adventure mode, 25*15/60=6.25 hours to beat adventure mode in melee with all characters.
If you could choose what character you wanted, and per character story line it took 10 hours to beat, it could take 240hrs to beat if there were a mere 24 charcters to choose.
I have to point out that this is a cinamatic game, my guess is that if it was in fact 240 hours, like 10 of those hours would be cutscenes, but you would be repeating a lot of cut scenes, so if you skip those it could be lower than they are saying.