Question about rare pet's spawn

How many hours after servers are back, after a maintanance, does it take for a pet to spawn the 1st time?

I know most rare pets spawn every 12 hours, but the 1st spawn it's not instantaneous, like with world bosses which are available most of the time as soon as the server is back.

I remember it was something like 5 hours, but I forgot totally, can someone tell me please? because I tried to buy the pet I wanted, but no one was selling it.