None of my books really give doses in terms of mgs or teaspoons, so I don't know. However, the one you have now has a MUCH higher mg of elderberry than the Nature's Way. Normally I'd say to follow the directions on the label, but I understand you're taking it for prevention/maintenance, and also want to conserve it to make it last through what's being reported as a REALLY loooooong flu season.
Here's an option, but I don't know if you'd be comfortable with it: take 1 teaspoon a day and see how that works for you. If, God forbid, you (or another family member) feels like they might be coming down with something, increase to 2 teaspoons a day or more, depending on how much is in the bottle, how lousy the person feels, if the stores have any elderberry, and where we still are in the flu season.
Heck, considering how much higher the mgs are in the Nature's Answer, you could possibly get away with 1/2 teaspoon a day. Or . . . Dee takes 1 teaspoon a day, everyone else gets 1/2 tsp.