What is the amount of time required to defeat the Masked Party God?

According to RealmEye:

To defeat him, players must deal 30000 damage in a set amount of time. He does not heal damage taken, but simply is unaffected and will continue partying if the required damage is not dealt.

You have to do 30000 damage in a set amount of time, but how much time do you have?

Without access to the source code, this will likely never be known. The Masked Party God doesn't display text when he heals like the Mysterious Crystal and other similar enemies, so the only way to find out would be a guild's coordinated trial and error or an extremely careful analysis of multiple failed Beachzone videos, calculating the damage rate from the displayed text.

If you were really curious, you could probably ask Nullbot (currently the most/only active dev) or another Kabam staff member.