If I have 100 000 xem, then how much I should get xem per months ??
Don't buy any looking to make money from fees. The chain is too empty. Treat it as a free bonus. You might only make 5 or 10 xem in months.
No even 5-10xem/month.
Fees are given to those who are active in the network (PoI).
See one of my accounts and the fees I harvested for being not active and having a relatively low importance score.