What might you suppose the computer is that spawns wild Pokémon?

Question

Is there a timer in each patch of grass that spawns one after a certain amount of time, or might there be a more complex one?

Answer 1

This is how it is determined:
> The rate of Pokémon encounter is determined from a simple mathematical formula:
1 in (187.5 / ( x )) per step

Let x equal the a value which determines how rare the Pokémon is. The higher the encounter rate, the more common the Pokémon is.

Bulbapedia

Comments

I hope this answers what you were asking :/

Answer 2

It's the coding. Like lets say There are Pidgey and Rattata
The codings are
3455-7866-8989 (Pidgey)
3455-7866-8980 (Shiny Pidgey)
1178-3462-5760 (Rattata)
1178-3462-5761 (Shiny Rattata)
These codings are fake for the example!!!
Ok, so pidgey is rarer that rattata. Then the coding will be randomly selected like this.
rattata
rattata
rattata
pidgey
rattata
rattata
shiny rattata
pidgey
rattata
pidgey
There is no timer! This is how Pokemon appear. So, you can walk one patch and encounter a wild rattata then turn twice and encounter a wild rattata, then when you are about to exit the huge patch of grass, you can encounter a Pidgey.
Hope this helped!

Answer 3

There is no timer for the wild grass. The Pokemon just appear randomly.
It depends how long you stay in the patches of grass.