the spawners seem to be immobile mobs, to be honest. I swear they 'alert' and 'see' you which triggers their 'attack' (spawn a mob). I could be wrong, but it sure feels like it.
Anyway, best to just stay near them until your weapons break or you get bored.
And as far as I can tell a spawner supports 3-5 before it turns back off. Something like that.

Warning: near but not TOO near. If you get too close the moronic rock throwers will bash their own spawner to death. Archers (skelly & drag) will shoot their spawners too, if you don't force them to hunt you down (I plant wood walls to break LOS and make them come around the side).