you would need enough 12V batteries with a combined rating of 350 Amp/hours + some extra. probably 450 amp/hours
To be honest this is a stupid question. you can't just hook up a whole mess of batteries to a UPS. That's not what a consumer UPS is for. Its to provide temporary power to save and shutdown. Your talking about a rack of 10-12 car batteries and a custom charger/controller/converter setup. If you have to ask this question there is no way you are going to be able to set that up
If you need power for longer periods of time out of a consumer UPS, buy one with better reserves. A 1500VA UPS would run for anywhere between 20 - 40 minutes with a 700w load (assuming that the 700w is not constant).
In the business world, a UPS is there to run servers and computers for enough time to either shut down gracefully, or supply power for enough time for the generators to kick in, and as most of them have line filters too, also to clean up the consistency of power to avoid spikes, drops, and other annoying problems.
A 1500va UPS is a 1500va UPS. Adding more batteries makes it something other than a 1500va UPS, so the question is not right. What you are really asking is how many va's or Kwh do I need in order to run a constant 700w draw for 6 hours.
Personally if I needed this kind of backup, I'd by a small generator with a large fuel tank (gas/deisel/petrol) that will run the 6 hours, and then you could invest in various switching equipement in order to start it when needed. Would end up much cheaper.