One would assume that you are playing with the latest patch/hotfix. If so, one should have at least 12 hours notice of a PP base being attacked. But that may depend on the difficulty level one is playing at. At Rookie level I am receiving a 24 hour notice of an impending attack.
From the Leviathan Patch Notes:
Phoenix Base Attacks
You will now have a minimum of 12 hours to prepare and respond to an attack on one of your bases.