When you have a dental retainer put in by a dentist the dental retainer lasts from 1 to 3 years and sometimes you'll need to replace the dental retainer within a year but sometimes they can last up to 3 years.
Be sure to lookout for signs of wear and tear of your dental retainer and if there is wear and tear or the dental retainer no longer fits then you might need to get a new retainer that fits.
If you want to keep your teeth aligned you'll usually need to wear the dental retainer for the rest of your life and will need to replace the dental retainer within 1 to 3 years.
It's a good idea to see your dentist and they can tell you whether your retainer needs replacing or not and can easily replace your retainer for you.
But if you've had the same dental retainer for 3 years then it's likely time to get a new dental retainer because it's likely wore out.