A good rule of thumb is that your stave will take one year to dry for every inch of thickness in its narrowest dimension. So if you leave your 6-inch monster 4 inches deep, expect it to take 4 years to dry. In my experience, the denser the wood the longer it takes, so for ash, oak, hickory and others that are more dense than average I'd suggest 1.5 to 2 years per inch of thickness.
If you want to use this wood soon, you should probably remove the bark, narrow the stave to about the width of the bow you want to make, and tiller this narrowed stave until it bends enough that you could brace it if you cut in nocks. Depending on how long your bow will be, this might leave you with 1/2 to 3/4 inches of thickness in the limbs, which will still take months to dry but it will go much faster than a full-size stave. You can speed the drying process up with heat or moving air (fan), but in my opinion this yields a lower quality stave than if you let it dry naturally for a longer period. I think that, particularly for more dense woods, the wood deep inside the limbs doesn't dry adequately when the outer wood is speed-dried, tending the bow towards reduced performance and excessive set. But that's just my opinion, perhaps others will disagree.
