Well obviously it's going to be different everywhere. There are far too many variables to come up with any sort of meaningful figure. But in any case I started to put every piece of junk mail that I received in my mailbox into an old postage box I had lying around. I forget exactly when I started but I believe it was leading up to Christmas 2010 in say late November 2010 and it occurred in an area outside of the Sydney Metropolitan Area in New South Wales, Australia :)
So roughly it's a years worth of junk mail - I say 'roughly' knowing it's more like 13 months, however I believe I lost some to the recycling bin due to it being left out in the rain and 'helpful' people.
Anyhow I'm sure you're curious to know how much was deposited.
...Well that box is full and that's a AA Battery to the bottom on the right hand side for some sort of scale for the size/room it occupies...or of course you could go off the bricks.
I weighed this (sorry, no evidence photograph you'll just have to trust me) using the highly accurate method of getting on the bathroom digital scales, waiting for the number to stabilize than held onto the junk mail - it weighed 22.5kg (including box...). That's 49.6 pounds - And that's a lot of resources.
Perhaps it doesn't sound like much to some but just think about how many letterboxes receive some sort of regular junk mail:
Sorry it's a bit blurry, but I think this photo sums things up. There are 19 mailboxes (in the first complex) with one that has a visible NO JUNK MAIL sticker - 8 rolls of junk mail reside in the others and it is assumed the other residents already retrieved theirs. Where those 18 mail boxes received 22.5kg of material, as I did, that adds up to a total of 405kg ~890lbs ....4 Ton worth where 180 mailboxes got the 22.5kg.
"Recycling just one tonne of paper saves approximately 13 trees, 4100kwh of electricity and over 30,000 litres of water"
- National Packaging Covenant by the NSW Jurisdictional Projects Group
One image I evidently don't have available to me right now is the recycling bin for private Post Office (P.O) Boxes provided by Australia Post. I also have one of these mail boxes...and every time a batch of flyers comes out that goes to PO Boxes, these bins literally overflow. I do have a picture somewhere which I can edit in when it is rediscovered.
If that isn't thought provoking enough consider this...
... fluctuations in weight exist during the year as there are spikes in who and how many business advertise this way. The above image is some pre-Christmas junk mail I got received and weighed. As you can see the roll of junk mail (and rubber band) weighs just under 1kg at 836grams ~1.84lbs
There exists on the internet a large range of information about the benefits of recycling paper but I won't delve into that aspect, but rather encourage you to consider them or look them up yourselves and make up your own mind on this issue.
Something I take into account myself is the time and other resources it took to produce the end product: From growing, nurturing trees and harvesting the wood which somewhere along the line becomes pulp of paper creation...as well as the combined distance it traveled including from the forest to the individual delivering it....to ultimately ending up in someone's mailbox before eventually getting tossed (hopefully) in the recycling or used for something else more useful than being landfilled.
It's a tough thing to comment on...Livelihoods rely on the industry, businesses need to promote who they are and what they provide, and people want to be kept in the loop of all the great bargains; whilst on the other hand it is pretty much a big middle finger to people interested protection of the environment and more sensible uses of natural resources, it promotes perhaps unhealthy purchases and consumer culture and it becomes part of the waste stream when the reader is done with it.