The company only warrants a shelf-life of 3 years, but unless you stored that tube in the hard vacuum of outer space for 3 years, the evaporation rate of silicone oil is so low as to be negligible. It's like peanut butter--if the components separate, just stir before using.
I should hope given the price it would be good for at least 3 years even out of the tube when applied between components.