There are two reasons for replacing an existing denture.  Either there has been a significant change in the hard and soft tissues or the denture teeth have excessive wear.

As a rule of thumb, if it is a first complete denture, I strongly suggest that the denture be replaced at least by the 5 year mark because of the accelerated change in the tissue during the first years of wearing a denture.

Replacement dentures many times are functional longer, but certainly 7 years is all I would recommend on most any denture.  There are simply too many changes that occur over a period of time.  Waiting an excessive amount of time before replacing the denture will usually result in increased loss of supporting tissue making it very frustrating for a patient to adapt to a new denture.