I've got about ten pounds of powder that is at least 75 years old. It came in a 50 pound, heavy wall, paper barrel. The barrel was one of 4 in a crate, on a pallet, stored outdoors but under cover so it wouldn't get wet at the old Hodgdon's storage facility.
The barrels and crate had some manufacturer's codes and dates with 6 inch high numerals to indicate what type of powder is in the container. 4831.
The velocities from the same rifle I purchased at the same time, over 50 years ago hasn't changed in a noticeable manner over all that time. The rifle is a sportered P17 Enfield 30-06 with it's cut off but original 22 inch bbl.
The load is 59 grains of 4831, over LRM primers under 180 grain spire points.
Back in 1970, bullets shot through a set of screens spaced 25 yards apart averaged 2830fps, around 20 years later, that same powder in the same cases from the original lot, loaded in a set of dies that hadn't been altered, using the same powder charge but a new lot of primers averaged 2815 fps over my first Chrony.
The last time I reloaded for and shot that rifle was 2016. The only thing that was different was the primer lot and I used a magnetospeed, which I had just acquired. I wanted to compare the velocities between the two chronographs I had on hand at the time. The average velocity on the Chrony showed 2835fps and the magnetospeed showed 2820. These averages were each from a 20 round count.
That powder was WWII or maybe Korean War surplus. Hodgdons sold it cheap enough that is was worth my while to drive all the way to Kansas and load up my truck with all that it could safely carry all the way home, sell it to local gun shops for resale and both of us make a decent profit.
There are so many variables to the OP's question that may cause velocity variations. Physical from solvent evaporation, vibration from moving around or even stored where there is a lot of vibration present, seating depth, difference in jacket material etc.
My tests are relevant for me, only because the 10k lot of bullets is the same, the cartridge cases are original to the firearm from when I first acquired it and the powder has been stored at room temperature, in the same magazine for over 45 years.
I know a fellow that complained bitterly about the life of his several cans of cannister grade powder. His preferred storage area for the powder was a cabinet, on the wall of a recip compressor room. It was dark, warm and dry and always at a constant temperature. His powders were consistent for a few years, then dramatically changed.
His tins of powder would get very dusty. The only thing we could deduce was that the vibration from the compressors, which you could feel on the wall and floor had caused the coatings on the powder to separate from the kernels. The temps in that room were around 80F consistently.
