Yes filling can fall off from already filled tooth and chances are more if one chews lot of chewing gums and takes sticky food. The duration depends upon the type of cavity, type of material used and the technique. If it comes out, one should get it refilled ASAP. For more on dental filling types and related FAQs you can browse the following link in our website... http://www.identalhub.com/articlecategory-fillings-19.aspx