General advice to those with highlighted hair is to go in for a touch-up every four to six weeks. Hair grows approximately one-half an inch per month, but if you're faster or slower growing, you may need to adjust those retouch dates. Some methods of highlighting can last longer, as the outgrowth isn't as obvious.
Consider that the type of highlighting you get may determine how often it needs to be treated. Ask your stylist before he puts in the highlights how often he suggests you come back, especially if you want to avoid frequent visits.
Not all highlights are blond; more subtle highlighting can be touched up less often. Blondes may be able to extend the six-week top deadline by using a clarifying shampoo once every few weeks to brighten it.