ok i have been a hairdresser for 13 years and honestly it depends on what colour you are, how often you colour it and how long your hair is.

i colour my own hair but with salon colour.

if you colour at home it is important to:
- use the same colour each time
-use enough colour (1 box if for short hair only or just a root touch up)
- don't try to colour correct (Blond + brown = Green)
-use a good salon brand shamopp to counteract the higher peroxide levils in home colour
- don't use bleach at home EVER
- if you mess it up don't try to fix it get help.
-do it ever 5-6 weeks and just a root touch up
-only colour ends every 2-3rd time

good luck hope it works out ok.