It seems to me that Kerrie at Mysteries in Paradise is worried about the size of her TBR pile. Therefore my question is: When do you really need to start worrying about it?
To help you in this sense you should not be looking only to the amount of it, but at your pile/reading pace ratio. To put it simply if you have about 50 books in your TBR shelf and your reading pace is about 50 books a year, your ratio is 50/50 = 1. So you do have about a year reading book coverage, which is my case by the way.
When do you need to start worrying about it?
After a thorough consideration I have arrived to a magic figure, 5. Once your ratio is over and above 5, which means you will need about five years reading to reduce your TBR shelf to zero, then you should start worrying about it and decide either reduce purchasing or increase your reading pace.
Simple and scientifically proved.
By the way you are still on time to cast your vote on her poll.