At TAR Brian Weatherston summarises a basic theory of forgetting.
S forgets that p iff at one time S knows that p, and at a later time, S does not know that p.He then lists counterexamples - for example, if S dies then S no longer knows that p, but clearly has not forgotten.
But the real suffering of this theory is that it specifies no concept of what it means to know something in the first place. Forgetting is an event that occurs subsequent to knowledge, so to ascertain whether something has been forgotten we must be able to tell whether or not it is known in the first place. This is a deficiency exploited by the other counterexamples he gives.
Obviously this is a little bit of a tricky question. But it seems to me that a theory of forgetting is entirely contingent on it.