If it follows the membership rate of this site - about 270 years!!
Technically 99'999'999 accounts, whereas one "user" is for guests and a few dozen "user"s are for bots. However, you could easily tweak this to 2'147'483'647.
phpBB has nothing to do with traffic if you mean the network. If you mean users being active at the same time then it's no problem for phpBB itself either.
I'm highly interested in seeing this in action - the board's content must be awesome.
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