How long should children believe in Santa?

Tweet

I was wondering if you think it's wrong to let them fully believe that there is a Santa Claus? For how long should you try to keep them believing? When they finally come to you with questions on whether Santa is real or not, do you think it is best to keep telling them there is a Santa until they reach a certain age, or do you think you should tell them the truth once they come to you and ask?