How long does it take to become a good critic?

So that's a new record for me. Writing 7 reviews in one sitting.

I reckon it takes writing around 100 reviews before you get the "hang" of it. That is, if you ever truly get the hang of it, which most of us don't think we ever do. At least it's likely to take writing that many before you can be consistently concise, get your point across, say everything that needs to be said, and of course - a most important discipline - to distiguish between what needs to be said and what you just fancy saying (because it's clever, or you noticed, or it's a bugbear, or..., or..., or..., - will the reader care as much as you do?)

Being a critic isn't mostly glamour and fun, what glamour and fun there is comes in exchange for taking care in, and caring about, your writing.