Prariedog, twice as long, or half again as long? Say, 45 minutes on 5ah battery, so 90 minutes on 7.5 or more like 1 hr on 7.5? I know I can get close to 1 hr out of my 5ah battery if I ignore the first red flash I see on the mower indicating low battery. I can usually squeeze another 10 minutes of mowing after I see the first red light.
I'm not exactly sure. We base it on what we get mowed on which battery. Using the SP heavily, cause we have hills, a 7.5 will do our big country front, side and back yards, and some of our paths or rough. The 5 only does one yard and the sides for sure. Sometimes we can squeak out a bit of the back if I stop using the SP. I also ignore the first solid red. Mostly cause I try to get mowed closer back to the barn so I don't have to walk so far to make the switch, lol. But because it can take a few hours to do everything, we bought a second so we can work simultaneously. With two of us working we get it done in about an hour, using the two 7.5s.