Hi!
There is no way to tell how long wood should dry before you make a bow from it, it depends on humidity, how thick the stave is, etc etc. If you have an accurate scale you can weigh the stave every now and then, when it stop getting lighter or with smaller increments you can try to make a bow from it. I use a moisture meter with 2 metal pegs, you insert the pegs into the wood and it gives you a % of moisture. For whitewoods it is ok to use the outside of the tree as the back of your bow. There is a lot of information on this site,...
greetings,
Stefan