There is a reason they call this "age hardening." Because the hardness changes with age.
It is hard to find age data for every conceivable bullet alloy, and it takes a long time to collect data -- every time I start an age experiment, I end up forgetting about it and eventually misplacing the sample bullets. Perhaps the age test could be accelerated by storing the sample bullets in an oven at say, 250°, to see how quickly various alloys anneal compared to WW.
Here is one of those age tests that I started and never finished. I was using my reloading press BHN tester which seems to read a little high, and straight WW, oven treated.
2 hours -- 22 BHN
1 month -- 27 - 33 BHN
2 months -- 34 BHN
3 months -- 31 BHN
4 months -- 33 BHN
6 months -- 28 BHN
9 months -- 19 BHN
12 years -- 14 BHN (old box of loaded ammunition)
air-cooled WW - 12 BHN
The burning question is, does HTWW eventually revert to 8 - 12 BHN, or does it stabilize at 18 - 21 BHN, as some people claim?
I don't know. I have heard and seen conflicting results.
Here is an age chart from the 9th edition of the ASM HT book (I think). Unfortunately, it only goes out to one year, and it is confusing to read because the time scale is logarithmic and the hardness scale is in Vickers units, but it shows the general trends. What they call alloy 415A would be similar to our humble wheelweight;