How longer & how much longer

"how longer" is incorrect.

you can use "how long" or "how much longer"...

how long have you been waiting?

how much longer am i going to have to wait?

How much longer is a mile than a kilometre? How much longer is the Rhine than the Thames?

I think that one would only see "how longer" in some tortured poetic construction, such as, perhaps: "How longer than a woman's life the great ideas that she proposes endure."

I suspect that you could live your entire life an never see such a locution used in English. I never have, and I've plagued humanity for well over sixty years.