At the moment, I have multiple reviews started, but nowhere near finished. Typically, however, I prefer to write my review all in one sitting. I find that my reviews are better that way.
As for how long it takes me to start a review, that depends on the individual item. There are some things that I can use once or twice before I feel comfortable writing a good review for it. Then, there are others that I need to use for weeks before I can write a review.