It has to to less than 7 turns practically - as base damage output from boss' girl cards is nearing 9000. That means even if you deny any single 3 combination for boss for successive Turns - you have to have HP accordingly (9,000 *6 turns = 54,000 and if you cant wipe out Boss HP on Turn 7 you will loose fight).
Increasing HP for top players is the latest forced trend as high HP is actually disadvantageous for newbie player (longer time for Tower Fight, tougher opponents chosen, longer time for Pussy Boat monster fight)