Six months is pretty standard. In most cases, the permit requires “start of construction” within 60 days, but if you’re not building it, then that definition does not really matter. The key is to move extremely quickly to market it for sale. The minute the permit hits your mailbox, you need to be hitting all the outdoor companies in your market with your sales presentation. It take most companies about 60 days to build a billboard, so that gives you a four month window. But even then, you need to allow for the time to write up the transaction if you find a buyer, as well as the usual excuses of holidays, etc. I would try and get offers in hand within 30 days of receipt of permit, so that you have plenty of time to react.