When do I say the bracha leisheiv basuukah?

Answer:

On the first night of succot one should say kiddush
standing up in the sukkah,  say the bracha
"leisheiv basuukah", sit down, and then say the bracha
"shehecheyanu". In accordance with the Talmud Yerushalmi
one should not say the bracha again during the rest of the
chag, the logic  being that the mitzvah of dwelling in the
sukkah is one mitzvah which extends during the course of the entire chag.

Alternatively, one may follow the opinion of the
Vilna Gaon which is that one should say the
bracha "leisheiv basukkah" immediately before
each time one sits down in the sukkah. The
Vilna Gaon's opinion follows the approach
of the Talmud Bavli.