Continuous lightning for over an hour; is this phenomenon common and does it have a name?
In Norwegian we call it kornmo (grain ripener); it happens in the late summer at a distance of more than 20 km so there is no sound.

The English translation is: "sheet lightning - heat lightning - summer lightning". I am not sure if this covers the meaning we have for it in Norwegian; it is said that Thor, the god of thunder, swung his hammer over the fields to get the grain ready for harvest.

This type of lightning is at more than 20km from the viewer and often 10-20 times this distance from you. It is really beautiful to see it.

This type of lightning happens high up in the atmosphere so one needs good viewing conditions to be able to see it. I do not think it is rare in itself; what is rare is the the atmospheric conditions that allow us to see it.

I guess this does not answer your question fully so I hope others post their answers too.