I think it's entirely possible to decide that something is not to your tastes within a bowl or two, but to really begin to understand a blend, it must be smoked in more than one pipe, and several times in each.
Reviewing a blend should be, in my opinion, more than just a "like or dislike" statement.
Every tobacco leaves some ghosts behind that will influence subsequent bowls to a greater or lesser extent. If the tobaccos are in the same "family," these effects can be minimal, but not even then, they're not insignificant. It can take several bowls of a new blend of a similar general type to really shine through and reveal itself more fully. (Of course, no one I know can devote a new pipe to every new blend, so we have to live with these little hauntings, and attempt to mitigate these lingering effects through repetition.)
I tend to devote a pipe to a blend for at least a dozen bowls if I really want to begin to learn its subtleties. Not everyone wants to do that. For the most part, we smoke our pipes purely for enjoyment, and it's not necessary to go to such lengths.
But, I often compare pipe tobaccos to wines. There are times when you just want to quaff something pleasant, and enjoy it, and other times when a more critical tasting is where much of the fun lies. In those cases, most wouldn't even consider "topping up" their glass with something different than what was already in it, yet many people routinely do that very thing with a new tobacco.
When I like a tobacco well enough to explore it more deeply, I'll devote the time to learning its nuances. Most of the time, though, it's enough just to enjoy a great smoke. If I were writing reviews, which I don't do, other than for my own personal pleasure, I would spend the time to get to know what I'm reviewing.
I have a deep admiration for the guys who write the review column for P&T. They not only have to spend time and repeated bowls on the tobaccos they like, they have to do to explore just as deeply the ones they don't. I can only imagine the scorched tongues they must sometimes suffer.