Winstrol suspension will separate, as will test suspension. How long it takes has no relationship to potency. It doesn't really matter because to get max effect you have to use it daily, and you need to shake the hell out of the vial before drawing it up anyway.
Also, it's especially important to be sure to wipe the top of the vial good and clean before drawing up every shot. Powder residue gets on top of the vial. You don't want that on the outside of your pin.