How long does one have to wait between thunder and or lighting in order to make another bracha?
Usually, one sees lightning and hears thunder more or less together, in which case one blessing (maaseh bereishis, according to the custom mentioned in Mishnah Berurah) is made over both.
However, sometimes one hears thunder, and makes a berachah, and later sees a (different) lightning bolt. Upon seeing the lightning bolt, a second berachah can be recited (either maaseh bereishis or shekocho u-gevuraso, but the custom is to recite the former). Likewise, if one sees a lightning bolt, and recites maaseh bereishis, one can then recite shekocho u-gevuraso on the later thunder.
Having made the respective berachos, one can only make the berachos again for a “new storm,” meaning that the clouds have dispersed, and then reformed into a new storm.
Sources: Shulchan Aruch, Orach Chaim 227, and Mishnah Berurah.