You can be on the keto diet for as long as you like or until you've reached your desired goals. It's really all up to you and your personal preferences and limiations.
Many dieters follow keto with a specific health outcome in mind like weight loss or diabetes control. Others choose to adopt it as a lifestyle living on keto for years on end. If you enjoy keto and your body is thriving even after you have achieved your goals, then there's no reason you shouldn't continue. Make sure that you are getting enough nutrients from all your keto food, including low-carb vegetables and fruits and you'll be safe.