Retainers are of two types : Removable and Fixed.
Fixed reatiners are fitted on the tooth surface facing the tongue, it is there in your mouth forever.
Whereas removable reatainers can be put and removed according to the patient's covenience.
Considering you are asking about removable retainers , these has to be worn for atleast 6 months to 1 year. The more time you wear them , the better the results can be expected.
These should be worn throughout the day , removed only while eating something or drinking anything else than water.
You can go to bed wearing them .
Your dentist will guide you about when can you stop wearing the retainers.
Next Steps
Visit a dentist and get an insight of the complete protocol.