Most yard trimmings will decompose in 6 to 12 months if the pile is kept moist through the summer.
You can speed up the process by:
Chopping up sticks and stalks,
Mixing “green” materials like grass with “browns” like fall leaves and stalks,
Making a bigger pile, about 1 yard square,
Turning the pile every month or two, and
Keeping the pile as moist as a wrung-out sponge. (Dry piles will stop composting).