The ideal amount of growth is a quarter of an inch of growth. This is normally three to four weeks of growth with no trimming, shaving, tweezing, etc. You have multiple phases of hair growth, and the idea is to have all stages of growth long enough above the skin surface to be able to have a clean, efficient service.
If you only allow for 2-3 weeks of hair growth, you have an increased chance of not having a hair free service. Short hairs may be left behind, or hairs that are on a different growth cycle may pop up shortly after the service.
Growing it out for the first service is always the hardest. But if you do it just this one time, it’ll last longer and be cleaner than if you rushed it. Plus, you’ll never have to worry about growing it out like that again with regular maintenance. After your initial service, the growth between services is substantially less. It becomes thinner, softer, finer, and less dense.