All ultimates have a cooldown period before they are ready again. I'm not sure if they are all the same, but the ones I've seen have been 20 seconds. If the warden is killed, he will respawn with a small amount of health depending on his ultimate level (40 for level 1, 50 for level 2...) and the ultimate will be put on cooldown. If he survives for 20 seconds, the ultimate will be recharged and will activate again on his next death, else he will be killed for the remainder of the round. This process can repeat indefinitely as long as he survives for 20 seconds after respawning to allow his ultimate to come back up. The ultimate will not activate if he is the last one alive.