What is the Halachah in practice?
The Mishnah Berurah (OC 227:5) writes that the common practice is to recite “Oseh Ma’aseh Bereishit” upon seeing lightning, and “she’Kocho u’Gevurato” upon hearing thunder, even though both blessings are applicable to both phenomena, as the Gemara here concludes. However, because both blessings are applicable to both, when one sees lightning and hears thunder at the same moment, he recites only one blessing — “Oseh Ma’aseh Bereishit” (if he recites “she’Kocho u’Gevurato,” he also fulfills his obligation).
If one saw lightning but did not hear thunder, or he heard thunder but did not see lightning, he may recite each one’s respective blessing (although there are various customs as to how to conduct oneself).
One must recite the blessing “Toch Kedei Dibur” (within a time span of approximately two seconds) of seeing the lightning or hearing the thunder. If he delayed, then he may not recite the blessing until he sees lightning or hears thunder again (Mishnah Berurah 227:12).
If the storm clears up completely such that no clouds remain in the sky, and then another storm comes, one recites the blessings again upon seeing lightning or hearing thunder, even on the same day (Mishnah Berurah 227:8). If the storm did not clear up completely, one may not recite the blessing when seeing lightning or hearing thunder again on the same day. On a different day, though, he may recite the blessings again.
For some more detailed discussion see Hirhurim blog