Long term use of SSRIs has simply not been researched. There's no definitely answer for how long you should be on them.

I was once described the guidelines for length of AD therapy as such: Around seven months for a first-time depressive episode after full remission of symptoms, two years for a second, indefinite after the third.

I do believe that untreated mental illness is far more deleterious to your brain than an AD. Research the facts: Depression and mental disorders damage certain parts of the brain and can cause a positive feed back cycle increasing the severity of the illness. Longer treatment on ADs before discontinuing them prevent relapse and thus prevent the destruction of mental illness on the brain.

However, if you don't feel completely better on an AD you may not be getting the protection against the effects of a prolonged illness.