The curl from a permanent wave (or perm) can last as long as year, if it is done effectively. During this time, the perm may grow out and change a bit, but the curls will remain intact. Some perms may last only 3-6 months, depending on the type of perm that is performed.
Perms can be executed traditionally using thioglycolic acid, or thio-free, which utilizes an amino acid called cysteamine. In addition to these types of perms, a digital perm may be done using temperature-controlled rods powered by a machine. The traditional perm is effective, but can smell quite strong. It is also more damaging to the hair. The thio-free perm has less of an odor, and causes less damage. It also produces a looser wave, and does not last as long as a traditional perm.
The digital perm lasts 3-6 months, and can recondition the hair, making it smoother after treatment. It produces a looser wave than the tight curls of a traditional perm. This perm can also damage hair, but is not as immediately damaging as a traditional one.
Perms can take up to 3 hours to be completed, but the result creates long-lasting waves that are even visible when the hair is wet.