Per week x For week: Como utilizar

Hi everybody!
I returned from desert "with my horse with no name..."
Depois de um longo tempo sem praticar inglês e sem trabalhar, estou de volta com muitas dúvidas.
Me perguntaram o seguinte: how much you earn per month? "
I was afraid when I heard that! Não seria "how much you earn for month?
More questions:
When I got sick, I was losing 3 kilo (per ou for) week.

please, help me! My brain isn't working very well.

"...in the desert you can remember your name"

Thank you