How much of a game should you play before review?

I am currently playing through Burnout Paradise with every intention to review it. With every other game I play through to competion of the main story and then have a dip into multiplayer and other modes. However with a game like Burnout the game never really has a diffinitive ending also with repetition of races you get a very quick idea of what the game is like but is that enough to review the game. Does anybody have similar issues when it comes to less linear games even RPG's?

I am currently playing through Burnout Paradise with every intention to review it. With every other game I play through to competion of the main story and then have a dip into multiplayer and other modes. However with a game like Burnout the game never really has a diffinitive ending also with repetition of races you get a very quick idea of what the game is like but is that enough to review the game. Does anybody have similar issues when it comes to less linear games even RPG's?

You don't have to play a game to 100% completion to write a review. That would be absolute nuts in a lot of cases. Just as long as you have a good idea of what the game is about, that is enough. Play it enough to where you can form a thoughtful opinion about each aspect of the game and that's all you need. Of course completing a game is ideal, it's just not always practical.

There's no point in playing through some shitty, boring, long or broken game to the end if it sucks from the start. Any reviewer can play whatever he or she wishes, there are no rules.  They should state in the review though, something like "I only played 10 hours and it made me want to chew my own eyebrows. Feel free to find out if it gets better after that".

And in the case of Burnout Paradise, you can play for 5 minutes and realise "Hey! This game is awesome!"

It's all relative

Until you feel that you've seen most if not all that the game has to offer, if a reviewer can, they should finish the game but there is no real rules.

Ideally finish it, but that's not always possible. As long as you understand what the game feels like and how enjoyable it is then feel free to write your review.

You should play as much of the game as you can (or want) until you feel like you can really put into words what you like or don't like about it.  If that's the whole game, that's fine.  If it's the first hour, I think you can make a case for it (after all, to paraphrase Eric Wolpaw, why should you have to play through crappy parts of a game in hopes that you get to the good parts?).