I thought I saw a pH pen at Mikes recently too. That could just be an old article. I actually got mine at Dollar Tree. I never thought about how well it might or might not work...at least not since I tested a couple things. The CM paper was acid-free and Aleene's Tacky Glue was
not! Big time!!

That was enough for me.
I thought it was really interesting that CM admitted at least one of their papers doesn't
stay acid-free after manufacture! Then what's the point?!?!?
As for the time frame question....I don't remember where I saw it now, but I was told this by a CMC also. CM tests their stuff in a special chamber that simulates over a hundred years. Again, I forget the exact number of years. One of their reasons for telling to use CM stuff is that while other products might be acid-free, the testing those products undergo isn't looking at long-term results.
I guess if you're making books that you expect your great-great-great grandchildren to be able to look it, then this is an issue. On the other hand, I'm doing this for me. I'm having fun doing it. DH has albums full of photos in special sleeves that are supposed to last forever. If someone wants to see the pix & my albums become yellow or brittle, they can just pull out his.
I could be wrong, but there's something about the CM mantra about their things lasting 100 years that tells me other acid/lignin-free things will be okay for quite a while too. You know what I mean? So it's 50 instead of 100...works for me.
BTW - I have scrapbooks done by my grandmother in the 1910s that are fine. I have some done by my great-great aunt that are also fine.
I also have a bunch of old photos that were stored in old shoe boxes (not acid-free) by someone else & they are fine. There were one or two boxes where some of the photos have weird spots on them....you can only see them when I scan them. I was told they're mold/mildew spots! Anyway, I'm fairly certain that this occurred when my parents had them in their basement.