No. Lord, no. I mean, unless we stop reviewing Final Fantasy games altogether, no way should game reviewers be required to finish a game. It's one thing to finish Mirror's Edge in an afternoon. With our industry's morbid obsession with game length, though, we've birthed some monstrosities that you just can't expect reviewers to actually finish.
Second, any game worth it's salt should deliver a representative idea of the experience it has to offer fairly early into the playthrough. If you -have- to play more than five hours to "get" a game, it's usually not worth getting. If your plot makes no god damn sense until the very end of a fifty hour game, that's the developers problem; they made a game that makes no god damn sense until fifty hours in. That's simply not a good argument for wasting a reviewer's time.
Last but not least, some games are just plain awful. I would not wish having to finish Kane & Lynch 2 on my worst enemy.
Now, there should still be some requirement on a reviewer; taking a five-minute glance at a game isn't giving any game a fair shake. However, when the only half sensible poll option suggests getting at least "the majority of the game experience", punctuated by a suggestion of "80%" in the original post, you've lost me. 80% of a run of the mill JRPG takes 40 hours to complete. That's a whole week's labor for a video game journalist. If we for argument's sake assume that journalist makes $20/hour, are you suggesting we demand video game sites like The Escapist spend $800 in wages to review "the whole game experance (sic)"? Are you for real?
If a game can't deliver a fair estimate of the game experience in between 5-10 hours, I don't want to waste my time on that game. I think it's unfair to expect more from reviewers.