The time it takes for a coconut tree to start growing coconuts after being planted is around 2½ Blockhead days. When it's only just started growing, it will produce 1 coconut, but after time, the tree will grow really tall producing a maximum of 4 coconuts per tree. Eventually, they will die and stop producing coconuts.
When you have 0 hunger, it will take 7-9 coconuts to fully heal your hunger bar. It is the same when healing from 0 health.