At those dimensions, a good 2 years. Rough it down closer to a bow dimension and you are looking at 1 year at the very least. The longer Osage dries, the better bow it will make. There are guys who dry Osage for 5 years to attain a premium stave. I won't even begin to make an Osage bow till at least two years of drying right after cutting down the tree. Three is better, four is sweet, and 5 is perfect.