I don't think that this is what DreamPlayer is looking for. This script requires the player to actually plant the herb themselves. I think that DreamPlayer is looking more for something more like in a DragonQuest game.
You may want to try something with variables. Like when you enter the area, a parallel process event randomly decides if the specific herb will be there. The more common the herb, the higher the likelihood.
The only issue is that it would not be constant amounts of time between appearances, and I also could not tell you how to set it up exactly.