Removable retainers come in a wide variety of designs. Two of the most common are the plate-type retainer (similar to the removable plates children wear, made of plastic and wires) and the vacuum-formed, aligner-type retainer. Plate-type retainers usually don’t need to be replaced unless they break, get lost or become deformed. The vacuum-formed retainer (a clear tray moulded to the shape of the teeth) is made from a thin plastic material that tends to ‘loosen’ over time. So, if there is no other reason to replace this type of retainer, we recommend renewing it every 6 months.
This answer is for general information. Your own case needs an examination by an orthodontist.