The Z-Shift happens in one turn. Victini powers up using Fightinium Z and then uses Power-Up Punch with 180 BP and Fire-typing that same turn according to turn order.
It would be interesting to make Z-shifting take up a turn without attacking, making players choose between Z-shifting to attack on turn 2 (perhaps with Z-shift-increased priority) and attacking immediately. For example, a player, on Victini's first turn, would have to choose between Victini:
A) Z-shifting {V-create --> power-up punch} for subsequent turns, but dealing no damage or effect on first turn --> risk of 1HKO or 2HKO if opponent is faster/uses (possibly Z-shifted) increased priority moves
B) Going straight to using V-create, taking the usual defense, special defense, and speed penalties, hoping that it 1HKOs the opponent before it can Z-shift (or 2HKO if Victini is > 1.5x faster and the opponent doesn't Z-shift increased priority)
C) Doing something else like try to status the opponent with thunder wave (the act of Z-shifting no longer will check accuracy and will skip other inhibitors like paralysis or taunt, but using the Z-shifted move will still check accuracy/paralysis/taunt/etc)
This would move some of the decision-making from the team builder to the battlefield.