Depends on the material of the screw used. Most cases where the screw is made up of a bio-composite material it is usually said to dissolve in 2-3 years time. In case of screws made up of Titanium, they usually can be removed but not before 2-3 years. until unless any complication due to screw like back out or impingement, there is no indication of screw removal all your life. Do not worry. they do not cause any harm if not removed.