How far can a fountain spread?

So, I got a bit of a weird question. Throughout deltarune, every fountain has been concealed, chapter 1 was in the unused classroom, chapter 2 was in the library computer room, 3 was in Kris' living room, 4 was in the church, and 5 was in Asgore's flower shop. Every fountain was put in a building or room that stopped it from spreading, but what if someone opened one outside in the open, where there wasn't four walls and a roof that stopped it from going any further? How much of the light world would the fou tain consume to make a dark world?