Can't say a definite time but it was probably a few months initially, then I went for OCD and sorted through pictures to get the ones with the oldest file creation dates (so that a Tiger desktop had the original file creation date for example). I also wanted them to all have the original file names, that was something some of the archives took liberty and changed the filenames. i wanted everything as original as possible.
Then it involved lots of time sorting through duplicates, almost duplicates that were very similar, etc. Used Automator to make a routine that upscaled all the images in Pixelmator and then cropped them all to 6016 wide, or if the original file was 5120 for 5k iMac I left it alone.
So far I think I have all OS 10.0 and beyond images and color swatches. All the OS9 I could find. And even a few desktop tiles from the OS8 days.
Currently, my desktop pictures folder is 10.99 GB and has 1062 images.