I usually put around 20 or so hours of game play into a game, just to understand all of its features, take a hard look at its graphical quality, understand its mechanics, adapt to the controls, get a feel of its game play...I can go on forever.
But more importantly, I play a game about 5 or so times before I write a review.
Five times is a bit extraneous. As someone who often writes reviews as more of a hobby than anything else -- just try to give an honest opinion about the game. Try to cover what is good and what is bad, and explain how the various mechanics of the game work together. If there's something particularly mindboggling good or bad, then you can focus on that, but still try to take everything into consideration. And don't worry about review scores.
Depends on the game. Quite a few games are rather short and do require multiple playthroughs to see and do everything you can see and do in the game. Most shooters are only a few hours long, so you can easily play through them twice in one day. I know some of my games here are only about 1h long.
And I guess you replied to the wrong person. While I do write reviews, I did that for a few years already, I know how I like to write them and I have no questions about it.