Maasrot, Chapter One, Mishnah Two
This mishnah lists various fruits of the tree and teaches when each of them becomes liable for tithes. The general principle seems to be that they are liable at the earliest point at which they are edible.
Since the words are self-explanatory I do not offer any explanation below.
When do fruits become liable for tithes?
1) Figs from the time they begin to ripe.
2) Grapes and wild grapes in the early stages of ripening.
3) Sumac and mulberries after they become red; [similarly] all red fruits, after they become red.
4) Pomegranates, when the insides become soft.
5) Dates when they begin to swell.
6) Peaches when [red] veins begin to show.
7) Walnuts when the nuts are separate from the shell.
8) Rabbi Judah says: walnuts and almonds, after their inner skins have been formed.