I'm honestly terrible at following other comic reviewers, and I should really branch out and look at what is out there (so I appreciate this post!).
There are a few I follow on tumblr, here are two (sorry I struggled to find the other few that I follow there for some reason):
Comic Thinking: super chill writing style, really honest.
The Webcomics Review: not quite a review website. More like running commentary and advice panels.
As for what makes a webcomic ready for a review, that generally depends on the critic's opinion. For me, I'd like the main characters (most, if not all) to be introduced, the world explained, as well as a good chunk of the plot developed (or at least leading somewhere). Usually having the first chapter or two out is a good estimate for a successful review.
Not all comics fit this criteria either. Slice-of-life, or gag-a-day comics typically don't have a structured plot. It is definitely trickier to review these comics, but I'd say have 20+ strips out for a decent review (the more the better!).
One issue you have to consider when requesting a comic critique is the waiting lists. Typically, these sites get a lot of requests. Depending on the critics availability, you could be waiting a fairly long time before you even get a review (just ask the people on my waiting list, haha). This might actually work to your advantage if you don't have enough content yet. You could request a review, and then by the time they get to yours, you (hopefully) will have enough content for a solid review.
Each critic is also different in the way that they tackle reviews. Some offer comparative examples from other comics (typically popular mainstream comics), some just voice their opinions, others may just summarize the story and then rate it out of 10, and you may also see reviewers grab examples straight out of your own comic as points of reference. Each has its own merit, but just keep these in mind when you ask for a review. What are you expecting to get out of these reviews? Can this particular critic offer what you are looking for?
Feel free to ask any questions about the reviewing process, or anything related. I'd love to help out where I can!
Oh, and here is my site if you or anyone else is curious.