Well one time thing in the sense that you're not constantly changing it, like you have to constantly add gas to a generator. Most people update things on their house like siding or windows etc at a certain interval. I guess 1 time thing is the wrong wording, but I just meant that you're not consuming anything to use it and it lasts a significant long time. You pay for it once for it's entire life time.
If you take good care of them they should last a good 10+ years. Ideally 25+ if you oversize the battery bank and never go below 10% DOD.
You need to buy good ones though, from what I've learned. The cheap Canadian Tire batteries I bought for my UPS around 7 years ago are slowly dying on me. They die with shorted cell or other defects. They were not really proper deep cycle ones though, I only realized after I bought the first 2 that they were a starting battery with deep cycle capability (Marine/RV) so I kept buying same ones to match them. I plan to upgrade that setup with some better batteries and the remaining ones will just go in my shed for the solar setup.
Lithium Ion may last longer and if yes that will perhaps make it more viable for solar, though but it's too early to tell for sure. We'll know when Tesla and other EVs have been around for 20+ years to see if they are still at a good capacity. That is a good sample for a worse case scenario too as cars see very bad conditions from very hot to very cold temps, and lot of vibration etc.
Battery tech is something we could use lot of improvements on for sure. Solid state batteries sound good but I feel we need something even better, those will be about double the capacity of lithium ion. We really need something that is like 10x. Essentially we need something that will make fossil fuel 100% make no sense. Why carry around all that weight when a battery has more kwh and is smaller. (if such a tech is discovered) I don't know what governments would do if such high density battery would be discovered though. It would completely kill the economy. It would be a win for green energy though.