Commentary for Nazir 55:8

Rav Ḥisda said: they taught this - that his days of leprosy do not count toward his naziriteship.

Only with regard to a short naziriteship - when he accepted upon himself a naziriteship of thirty days, and contracted leprosy after he had done ten days, and was confirmed a leper -- those days do not count for him. Because if you say they do count for him, not enough time will remain for him to grow his hair after the leprosy-related shavings.

However, with regard to a lengthy naziriteship - [such as] where he accepted one hundred days upon himself, and contracted leprosy during his term of naziriteship before thirty days had passed, and was shaved with the two leprosy-related shavings -- since there will still be thirty days remaining for him in order to grow his hair, even the days in which he was confirmed [a leper] count for him, and he need only keep thirty days following his leprosy-related shavings, in order to grow his hair for his naziriteship in a state of purity.