That's similar to what I'm already doing. Each turret has a 4 second timer which, when it expires, triggers the turret to scan for a new target. As the level is loaded, each turret's timer is set to a random offset between zero and four seconds.
But your way's actually better, since it sets a maximum number of turrets to process, and doesn't allow it to be exceeded, whereas my way is prone to "clumping" of timers expiring.
But your way's actually better, since it sets a maximum number of turrets to process, and doesn't allow it to be exceeded, whereas my way is prone to "clumping" of timers expiring.