I know a guy who reserved a number for twelve years before his
airplane flew; just keep paying the fee. Your state may contact you
if they register/tax aircraft, but every such state I know of exempts
homebuilts under construction; they'll require you to contact the
state office within a certain period after the FAA issues the
airworthiness certificate. (Save all parts/materials receipts to
establish the tax basis or registration fee. Some states also factor
in your labor hours -- at some pitifully small rate -- so you may have
to show building-time records, too.)