I'd think spending 8 - 10 hours with a game would usually give enough material for an in-depth review (although certain MMO's may take considerably longer to get the feel of). At any rate, if the reviewer feel that the game might have more to offer than he's had time to find, then he can simply inform in the review that it's based on X hours of play time.
While it would of course be ideal to finish a game to review the complete experience, some games cannot be finished, and others take 100+ hours to get around all the nooks and crannies in. And something like production values, gameplay, and characterization should be quite apparent by the 10 hour mark in pretty much any game. And who wants to play through 10 hours of horrific storyline, even if it becomes noticeably better in the last 10?
Overall, I think reviewers are able to form an opinion based on a reasonable amount of game time, although there maybe certain niche genres where playing through the whole thing is arguably paramount (such as Heavy Rain, due the extreme focus on storyline in that game).