I don't know how long these will last used constantly .. my RV deep cycle lasts maybe 2 season, but that is used lots. An occasional use, I would guess 3-4 yr is a good av. battery life.
As to how long the unit will power your pumps ... depends on the battery & the curciut. In theory, a 30 A-H battery should have 3 A-H at 120V, which should power your 2 pumps (total of 20W or so) for 25 or so hours. <I found a site for a B&D 30 A-H box that said 30 hrs at 25W...>
If you need longer, a DIY version is prob better. Eg. my RV deep cycle is 100 A-H <Costs ~150>, add an inverter & a charger <~60-100> & you have a setup that is at least 3x the capacity of the typical powerbox. And you can just buy more bateries if you want a bigger system.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.