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.
A Crime is Afoot 
>Using your formula, my ratio is actually pretty manageable, so I think I'm just not going worry about it and buy a few more books. 🙂
>Aunque siempre he pensado que mi lista TBR era infinita, que para eso hablamos del plan infinito, estoy más tranquilo desde que leí que Google ha determinado que el número total de libros existentes, actuales y pasados, es tan sólo de unos 130 millones. Así que el plan infinito ha quedado, por fin, acotado. Más en serio, mi TBR era habitualmente de unos 20-25 libros cuando dependía del libro físico, tanto por razones de espacio como de inversión. Pero desde que tengo el Reader la lista se ha disparado y actualmente está totalmente fuera de control.Saludos
>I can't tell you how relieved I am to know that my TBR only covers the next 15 months of reading. I'm so happy I may go out and buy more books!
>Karen, Naomi, am glad you can add to your pile without finding you guilty about it.
>Fab, regala el 'Reader'. Yo ni lo intento.
>I like your ratio Jose Ignacio – I've only got about 18 months worth of reading so I'm fine 🙂
>I think a ratio of 10 or 20 is more reasonable (and thus eases my conscience!). As long as you still have room for life's essentials, more books means literary security. Which is far more solid than financial security!Seriously, I'm taking a new tack and giving away books I've read, so that only TBR's are in residence. I'm trying to stop considering books as some weird kind of trophy.
>Er, I daren't think about it but 5 is about right for me, sometimes it goes up higher than that, especially on the occasions I meet up with Karen! (My total does not always count the books that arrive unsolicited in the post, though!)
>At your reading pace Maxine I'm sure you can take some leeway.