When you have teeth whitening done professionally it's recommended to have the teeth whitening done professionally every 6 months to a year.
I only have the teeth whitening done professionally every 2 years which is more than enough for me but some people get the professional teeth whitening done every 6 months which is the recommended interval.
But you don't have to have the teeth whitening done that often if you don't want too or cannot afford too even though it's best for most people.