to be honest, I don't think anyone or any site knows what's the exact respawn rate..
but if you want money, I have an Idea:
in mount moon a man (entry floor , extreme left where there is a ladder) gives you 5 or 10 pokeballs if you don't have any pokeballs. go sell them and come back, he'll give you 5 or 10 balls again!
(you know what to do now, right?)