The first I did experiment was digging an ocean, planting a forest, putting down a fruit tree and then waiting. After the settlement, I marked the borders by putting down a Reus and then regularly checking to see if they'd expanded. They did, with an interval somewhere between 3 and 4 minutes, with no apparent bias to left versus right. This was on a 30-minute world.
Anecdotal evidence, because I still have to try the following varying factors:
- Longer world. I feel the rate should slow down for 120-minute worlds, but I haven't tried that yet.
- Neighbouring biomes: does it take longer to go into the ocean, say?
- Local availabilty of prosperity: does a village with two food grow faster or slower than a village with 80 excess food? That could be tested with some extra elder/strawberry combos.
- Total availabilty of prosperity: does it make a difference if a village half the world over has way too many resources?