It's explained that the grail will materialize before it's fully "filled", but it's only complete once all 7 heroic spirits are dead. From the Novel when Kariya hands over Irisviel to Kirei:
“To be precise, it is this homunculus. If one or two more Servants are finished, then it will probably show its true form… I will prepare the ritual to receive the Grail as it descends. Until that time, let this woman also be temporarily under my protection.”
And later, while Kirei and Kiritsu duel:
Directly above the big prop warehouse where the two men were, Irisviel’s already-cold corpse was placed upon the raised stage of the music hall. [...]
After Archer’s victory, this vessel had finally absorbed the soul of the fourth Servant. [...]
The corpse of the beautiful homunculus was completely consumed by the heat in the blink of an eye, and was reduced to ashes. That was not all. The golden cup that had contacted the outside air charred the floor and the curtains, and roaring flames enveloped the completely empty stage.
On the stage where the fire was raging ever wilder, the golden cup floated in the air as if it was upheld by a pair of invisible hands. The ceremony of the descent of the Holy Grail, which the Three Noble Families of the Beginning had desired so much, had begun silently even without the presence of a priest