I don't understand what you mean by the word "supple". I don't think of wood as being supple, and it certainly doesn't become more supple as it seasons.
In terms of seasoning, the thicker the piece of wood is in its smallest dimension, the longer it will take to season (which basically means fall to a moisture content that's at equilibrium with the air).
One way of speeding the process along is to rough your bow out to floor tiller stage, about to where you can brace it. Then let it season while clamped to a 2 x 4 or something so it doesn't warp. Once roughed out, the bow will only be 1/2 inch thick or so, so it should reach equilibrium moisture content in 3-6 months, depending on the humidity. You could also put the bow & 2x4 in a hot box or kiln to force the process along. That can dry it in just a few weeks or even a few days, depending.