You'll get all sorts of answers to this question. If you leave it as full-dimension staves, I'd say at least a few years. A rule of thumb I've heard tossed around that seems to work well for dense woods like hickory is one year for each inch of thickness in the thinnest dimension. If you reduce the staves to rough bow dimensions, you can speed this up somewhat but I'd still say it would be ideal to wait a couple years before you use the wood. Wood will reach equilibrium with its surroundings and be 'dry' before it will be seasoned. Seasoned wood is better to use for bows, if you have the option, though plenty of effective bows are made from wood that is merely dry.