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In this article it's mentioned that one very important feature of the LEGO bricks are their 'clutch power' which is "the ability of its bricks to snap together tightly while also being easy to separate, thereby readily allowing for de- and reconstruction"
Since a LEGO brick is ABS plastic and will wear with use, I wonder how many times you can assemble and dis-assemble two pieces before they no longer stay together.
Well I did it. I built a machine to test this. It took 10 days until the LEGO at the bottom couldn't stay on anymore.
The optimal clutch power is attained after 8 to 10 couplings(search for "clutch"); that is, before that, the bricks will sometimes cling a little too much on each other.
However, I don't think anyone ever actually stated when clutch power would start to deteriorate. I'd tend to assume that once the optimal clutch is reached, there is just enough plastic at the coupling to make it work, but not enough that it would wear of.
Of course, as you said, I suppose there is still a little that wears off, but I'd wager it isn't that much, and it's probably negligible with regards to the manufacturing tolerance of the bricks. In other words, you're more likely to find two bricks which come loose too easily because they're at the limit of the tolerance, each at a different extreme; than to take two bricks with normal clutch power and try to wear them off so that they too come loose easily.
Then again, I don't think it's ever been extensively tested. We'd need two bricks, and a robot that assembles/disassemble them a great many times. (Unless you want to do it manually)
I'm not sure there's any official answer for this, however I can answer with an anecdote.
There have been a few times where I've come across old bricks that have been assembled together for a very long time (ie many years). When prised apart, I've spotted that the studs have actually made an indentation on the sides of the tubes.
Based on this, I wonder whether there's more danger of bricks losing their clutch power from being assembled (which means they're in compression) for a long time, rather than being repeatedly assembled and dis-assembled?
