Can I prepare a cornstarch slurry in advance?
"Can I prepare a cornstarch slurry in advance and how long will it last?"
Making a slurry out of cornstarch, which you do in order to add the thickener but not lumps to a hot liquid, is a simple matter of dissolving cornstarch in a little liquid (most often, water). The cornstarch dissolves very quickly, which leads me to answer your question with a question—something my mother told us to never do. Why do you want to make it in advance? It wouldn't be much of a time saver, both because it takes less than a minute to dissolve cornstarch.
Whatever your reason, I don't think it would work well. If you make the slurry with equal parts liquid and cornstarch, you will need to give it a little stir after it sits—so you stir twice instead of once. If you make the slurry thicker to avoid that, you may run the risk of it drying out and hardening.
Please don't let this discourage you from looking for ways to become more efficient in the kitchen—and by all means share your ideas here!
This Chicken-and-Veggie Stir-Fry recipe is a great example of using cornstarch as a thickener for the sauce part of the stir-fry. And it only takes about 1 minute for the mixture to thicken.