If you relax your hair at home or go to a stylist, the recommended time that most agree on is every eight weeks. This gives you enough new growth to apply the relaxer to; it may vary from 1/2 inch to 1 inch.
The most important part of touch-ups to remember is that you don't apply the relaxer all over the hair. Previously relaxed hair is permanently straightened -- it doesn't require additional chemical processing. You're only applying relaxer to your virgin hair. If you apply a relaxer to hair that's already relaxed, you'll most likely experience breakage and damage due to overprocessing.