The runtime depends on your current draw. What exactly is it that you are trying to run? Also, keep in mind that alkaline batteries may not be a good choice here. Alkaline batteries are not designed for high current. And as current draw increases, capacity decreases drastically. So although it would seem like you would get half the runtime at twice the current, you actually get much less than half the runtime. Also, if you are trying to run something that draws a high current, alkaline batteries simply can't do it. For instance, three amps just isn't happening with alkaline batteries. And finally, voltage drops drastically as current is drawn from alkaline batteries. So lights tend to dim rapidly with alkaline batteries. This is especially true with incandescents, as the light rapidly turns orange as voltage drops.
The best thing to do here is get some good NiMH batteries (Eneloops, Duracell Ion Core, etc). Not only are they better designed for high current draw. But they hold their voltage better. And, of course, you can recharge them. That way, you don't have to be so concerned about wasting batteries.
