Porcelain and composite veneers are permanent because a thin layer of enamel is removed from the tooth to install them. With proper care and regular dental checkups, veneers usually last a long time, up to 20 years in some cases.
They only need to be replaced if they break. If they become loose or fall off, save the veneer, and your dentist can reattach it without having to replace it.
There are also some newer options that do not require the removal of the enamel, but these do not look as natural. These options do make it possible to remove them should you decide you no longer want them.