I'm gonna have to say it depends on the dye. If its the box color dyes like brown, black, brassy colors, stuff like that that you can find at Walmart, I'd only touch up maybe once every 2-3 months, box dyes aren't the best for your hair. If your using color stains like purple, blue, pink, withthe dyes that are from the brands like splat, manic panic, punky colors, or similar stuff, as often as you'd like. If its like blonde colors from anything, or bleach turn I'd say once every 2-4 months with lots of treatments.
You can do it as often as necessary... Though they do advise that if you do too much too often, there could be a chance that your hair could break... Especially if you're dealing with bleach or blonder colors.
Read some boxes of hair dye and see if it is safe and how many months it last, and stuff like that..