Silicone does not "dry out" over time. It was designed to be used as an all weather caulk and as such it's life expectancy outside is 35 years or more. I have had tanks sit in an unheated garage for 10 years and hold water just fine for another ten years. If the edges of the fillet seal are starting to lift or anything is visible underneath it, a reseal is in order. The tank does not necessarily have to be leaking. A properly built tank will hold water without the inner fillet seal. It is basically insurance.