As
@Newdude points out, this is not explained well enough in the manual. But I take it to mean you've used
at least 80% of the cell's life. Or somewhere between 80-99%. As I mentioned, I would start thinking about a replacement, and keep whatever is left in the old one as a backup. You'd use the new one while it is under warranty.
More importantly, I can't imagine the price of this thing going down any time soon (read: not ever), and might be one of the many products that are going to have a huge price increase in the next few months (just my opinion). They might also be subject to supply-chain issues, which might mean not being able to get one right away after your current one poops out. For all those reasons, if it were me, I'd buy one right now.
Or think of it this way. An SWG has all the chlorine it can produce "stored" inside. But unlike liquid chlorine, it doesn't degrade in a few months. You'd be buying your future chlorine at today's prices. Sure, it's speculation, but, IMO, I think it's a sound investment, one that potentially could have a really great ROI.