I know, I know, I should be making my own eggnog, but when I'm in a pinch and I don't have the time to make my own eggnog, I have to rely on the store bought stuff.
With that being said, can anyone tell me a standard ratio of eggnog to brandy/rum?
There is no standard, it all depends on how happy you want to get, and whether you just want the flavor of the booze, or the actual effect of the alcohol. Also a factor is the kind of alcohol you want to add. Light rums, for example, have less flavor than dark rums, so you'd want to use dark rum if you want booze flavor over alcohol.
As an estimate if you are going for flavor an ounce or two per liter (or quart, they're close enough for jazz) will do the trick, or up to 6 or 8 if you want very happy people.
Don't forget to grate a bit of fresh nutmeg on top before serving!