What brand do you use and what is the NPK ratio, if any?
I use Spray N Grow.
I still have some left over from 3 years ago. As long as the container is sealed properly it will last for a few years; however, once the Spray N Grow concentrate is mixed with water, the resulting mixture loses effectiveness in 4-6 hours.
This may not answer your question about the shelf life of your favorite liquid fertilizer, but at least you will have some idea.