A perm, or permanent wave, is a way to chemically alter the structure of your hair so that the hair changes its shape and morphs from straight to curly. Because the change is semi-permanent, it lasts longer than just using rollers or a curling iron, but a perm doesn't last forever.

The effects of a perm usually last from three to six months, depending on the kind of perm you choose, the condition of your hair prior to the perm and how you maintain your hair after a perm.

How long your perm lasts also depends on whether you perm your hair at home or have a professional do the job. Because professionals have access to stronger chemicals that aren't available for home use, perms done at home have a shorter lifespan than perms done in a salon.

Ideally, you should perm your hair whenever its effects start to fade if you want to maintain the look of your perm.

There are several different types of perms. Some kinds of perms, including acid perms and exothermic perms, create dramatic results that need to have their roots permed again every three months to maintain the effects. Gentler perms, including body waves and stack perms, are more subtle and can often last four or five months without needing a touch up.

You can often extend the life of your perm by adequately preparing your hair before your perm appointment.

Hair that has been deeply conditioned or that has a heavy silicone build up will absorb less of the perm solution, meaning you have to perm your hair more frequently to maintain curls. You can make your perm last longer by washing your hair with a clarifying shampoo and avoiding the use of conditioner for 24 hours before your perm appointment.

Damaged hair is also less absorbent and more likely to be damaged by perm chemicals than healthy hair. If your hair is dry and brittle and it's time for your perm appointment, talk to your stylist about non-perm options to tide you over until your hair is in better shape. In some cases, your hair may need as long as six months to a year of recovery time between perms.

Using shampoos and conditioners designed for use by people with permed or chemically treated hair will also make your perm last longer.

If you're unhappy with the results of your perm, you can transition back to straight hair by making a plan with your stylist.

Though you can simply wait for your perm to grow out, the difference in texture between the top and bottom of the hair can create an effect that many people dislike. A more subtle transition calls for gradually reducing the curl every four or five months with increasingly gentler body waves.

The chemical used to create curls contains peroxide, which can subtly lighten hair. Because of this, you should wait until after your perm has settled—usually at least 48 hours—before highlighting or coloring your hair. Check with your stylist about how long you should wait before coloring your hair after a perm.

Because perms are a chemical process, you shouldn't combine them with another chemical treatment, such as coloring or relaxing hair.