David, The rule of thumb is 1" per year. You could cut a strip off of the board, take it down to near bow size and have a bow in a few months. With a hot box or similar dryer maybe even sooner.
What is the dimensions of the boards you have? You could cut it into board stave size(1"x2") and stack with stickers to prevent twisting but allow good ventilation and when one is ready, they all are. If the wood was green when you put it up, you might want to wait a bit more before cutting all of it. ..but one or 2 strips for bows shouldn't hurt. Pat