I think as long as it is stored properly nitro fuel has an close to unlimited shelf life. I have never seen any fuel with a sell by date or anything like that on it. The trick is to store it off the ground in a stable climate with limited exposure to light. My understanding is concrete or metal can leach nitro through the bottle somehow. I dont understand how that is possible but have heard it enough times that I have the beleive it to be true.
I have some fuel that Ive had since mid summer. I store it in my closet on a wood shelf. So far so good.
The signs your fuel may have gone bad would be a seperation of the dye or oil or some sort of discoloration.