I can't speak for music reviews, but I used to work in a NYC screening room co. where movie reviewers watched new movies. Almost all of them watched the opening credits while talking on their phones or writing etc, then watched the first 5-20 minutes of the film and went out to write their review. Many, many times they asked the projectionist (me) how the film ended & if I liked the film. Many, many times I later saw my comments in print.
These are top film reviewers, too, whose names you recognized at the time - I've been out of the biz for quite a while, don't pay attention to film reviews any more after that.
There were a couple of guys who watched every movie all the way thru, every time. Don't remember their names, one worked for the Village Voice and the other for The Atlantic, I think? I loved reading their reviews, it showed that they really worked hard and had valid opinions.
BTW I agree about the repetition thing, I never heard of a movie reviewer watching a movie more than once before writing a review - and in the projectionist biz, we watch movies over and over, so much so that we call the first time the "practice" watching because you miss so much the first time thru.