The length of lock out doesn't bother me.
Why not have world bosses spawn every 15-30 minutes, but put a lock out on players who have already done it? The lock out could be 12hrs, 24hrs, a week, whatever you like. Making world bosses only spawn every two hours or so is incredibly frustrating, especially to more casual gamers, or gamers who might even play regularly but can only gather enough friends to do a world boss at a certain time.
