Every case is different. Some people need to wear them every night, while others will only need to wear them for limited periods of time. Some people find that unless they wear retainers all day, every day, their teeth want to move back to their previous positions. If after 6-12 months, you find that your teeth are still moving when you don't wear your retainer,  you may wish to see your regular dentist or orthodontist to get a permanent retainer made (or a removable, but more 'heavy duty' one). These tend to last longer than the plastic trays, so if you're going to be wearing them forever, they might be a better choice.