At the beginning of the series, we saw that
Monsignor Pruitt walked around
during the day without any trouble. It was days, at the very least, if not weeks, before Pruitt's condition progressed to the point that he could no longer tolerate sunlight, and he did not exhibit as much loss of control as most of the others.
However, Riley exhibited extreme sensitivity to sunlight mere hours after drinking the blood of the winged vampire. Similarly, the townsfolk who were transformed in the final episode turned to ash upon daybreak.
As such, why did Pruitt change so slowly?