It's going to start changing immediately and continue to change for decades IIRC. Whether you have the instruments capable of seeing the change is another thing. Bhn is just a number, the load either shoots or it doesn't.
True dat. However, it's important number when you're trying for the softest (hunting) alloy that will shoot in your particular firearm, with little or no leading. Gas check or not.
Over the years, I've come to the conclusion that pure lead normally yields the shortest tracking distances. I want my hunting bullets soft. Whether they be solids or HP's. Pure lead is too soft for centerfire firearms............running the risk of leading.
Since I have my own range, I recover almost all of my spent bullets, every year or so. Now, it will be a combination of the preceding years Air cooled
Recovered
Bullet
Alloy (RBA), pure (from muzzle loaders), linotype (mixed with pure) & 20-1 alloy. BHN will vary from year to year, depending on how much of the different alloys are shot. BHN will range from 13-15, depending on how much pure is deposited in the berm. Personally, I like to know the current BHN.
Air cooled RBA alloy (13-15) is used for carbine, revolver or pistol practice/plinking. Never for hunting. IME, it just acts as a FMJ, poking a caliber size hole, resulting in extended tracking. With a BHN tester, I can cut the RBA with pure to the point that might meet my criteria for a relatively soft hunting alloy, that can be duplicated.
Occasionally, I will heat treat my RBA for use in my AR-10 carbine.................so I find BHN a most useful tool. YMMV