This depends on how many food scraps you use and what kind they are. If you bury the trench in early spring, you will likely be able to plant the following spring. An easy way to test if your area is ready to plant is to dig down to the level where the scraps are buried. If the scraps are unrecognizable, then you're ready to plant. If they still look how they did when you buried them, they need more time and maybe more water.