how frequently do we say the bracha on thunder?

Generally speaking, we can only say this bracha once a day because all of the thunder that we hear in one day is considered to be parts of one overall thunder. However, if the clouds completely cleared but then came back that day, resulting in another round of thunder, then this is considered to be a 'new thunder' and we do say another bracha for it. (Mishna Berura 227:8).