To reiterate what Luca and James said – regeneration is based on a chunk, which is a column of 16 blocks by 16 blocks and the full height of the world. The regeneration described is for the standard blocks, but not “resources”, which are all the seams, the boulders, the fungi and the plants. The “resources” all regenerate (in one go currently) as soon as a chunk has regenerated all the standard blocks. The types and positions of resources in the chunk are randomised based on how many of a given resource there are in the world and various other conditions (altitude, block types etc.).
So the resource regeneration is a separate process to the base regeneration, but is completely dependent on it. So when there are areas with no resources it is normally because of the base regeneration not finishing. So these changes will hopefully make the regeneration of the world feel and look better, so if a huge castle gets unbeaconed it will decay over many days in a satisfying way, as well keep resources stocked because chunks aren’t blocked from being regenerated by players moving near to them each day.
In terms of time taken to decay, this is the proposed balance:
10 block changes will take 2.5 hours.
100 block changes will take 24 hours.
1,000 block changes will take 114 hours.
10,000 block changes will take 131 hours.
20,000 block changes will take 138 hours.
30,000 block changes will take 145 hours.
50,000 block changes will take 158 hours.
Bear in mind that is per a chunk, which has 65,536 blocks in it, so the chances of getting to the higher numbers is very low. When I looked at existing builds the highest number of block changes I saw was 1,500 per chunk and for most builds it was much lower than that.