Suture removal depends on the material and extension of the procedure. The stitches should stay for a period of 7-10 days. Some stitches are resorbable and don't need to be removed. Generally, the dentist explains it to you after the procedure is completed.
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Stitches that don’t fall out require a follow up appointment with the Dentist that placed them. There can be exceptions, follow up with your Dentist.