I am in the process of collecting all US, Australia, and Canada BOB stamps. I have about 200 collected so far.
One could spend several lifetimes (and millions of dollars) just trying to complete back-of-book for one country, let alone all three. I've been collecting just U.S. revenues for going on 2 decades now, and have just a tiny fraction of what is even just listed in Scott.
I guess how deep you go into B-O-B makes a difference as well. There's a world of non-Scott-listed material out there as well.