I don't have those mics, or that battery box, but I think the answer is probably "quite long". The little brochure that came with my DPA MPS6030 (9V battery box), says "With two microphones: Approx. 300 hours (with alkaline batteries)". Maybe a little marketing in there, and maybe the Countryman's draw more power than 4060s, but I have definitely put ~80 hours on a battery.