The general recommendation is to buy new fuel every year. However, this doesn't make sense for disaster preparedness. In addition, it can waste a lot of valuable fuel. This recommendation is typically for someone that relies on their fuel daily so that they can more beneficially manage their supplies.
There are additives that you can purchase that will extend the life of your fuel for 5-10 years. One of these is called PRI-D. It is a diesel fuel additive that works well on kerosene. The cost is around $18.50 for a 16 ounce bottle that will treat 256 gallons of kerosene.
You don't want to just pour the additive into the tank. It should be mixed in. That means that you can stir the tank, shake it, or use a siphon to put it into another tank.