I'll make this as short as I can.
I dyed my hair black 6 years ago, and it turned white. Now I don't know if it was from the dye, or because of the fact that I was dying my hair every month, but some of my hair turned white. Now after 6 years, I just recently dyed it black a month ago, and most of it has washed out.
My mom told me I am not supposed to be dying my hair every 4-6 weeks, because the dye will damage my hair. She wants me to dye it every 6-7 months. The problem is A) I can't wait that long, B) Most of the dye will be out by then.
So here are my questions:

1. How often should I really be dying my hair in order for it not to turn white or fall out?

2. Is there any way I can keep the colour lasting for as long as possible?

Thank you.

Depends on which type of dye. And yes if you need to touch up your roots you can color your hair. You don't need to wait 6 months for a touch up.. that's ridiculous.

Are you using permanent? If so it can fade with time.. most dyes can.. Permanent is your best bet though.. But don't let your mother fool you, you can color your hair more then once every 6 months.

Your hair grows about a half inch each month... Its best to color your hair every 6 weeks or so.. in the summer months your hair grows faster, so keep that in mind.

Check this website.
How often should you color your hair? - by Laurel Garrett - Helium (http://www.helium.com/items/297378-how-often-should-you-color-your-hair)