My upper lateral incisor inside the crown has infection. the dentist did a root canal and it is still paining. he said that ittakes 3-6 months to heal because of the big cyst. is it true?

Yes and...: It could take that long to heal.. If by healing you mean for the bone to fill in... If it gets worse i would go back and see the dentist.

Cyst or abcess?: Endo treated teeth can feel uncomfortable after therapy but usually subsides after a few days. Discomfort should steadily decrease in intensity and frequency. The level of discomfort needs to assessed and addressed accordingly. If you are still as uncomfortable a week or so after tx. As you were before tx., then tell your dentist and/or get another consultation.

Also,: I am in full agreement with my colleagues. However, not all root canal therapies are able to resolve large infections, especially when there is a true cyst involved. You may need to have a surgical procedure known as an apicoectomy or possibly even have the tooth extracted if the rct does not resolve the problem. This needs to be carefully monitored. I suggest monthly recalls with an x-ray.

yes can take timr: Yes..Can take time to heal if had a big cyst.