In that case you’ll want to keep the bottle in a cool dark place to try to preserve it as long as possible.
I’ve found the effectiveness does start to drop off after around the 6 month mark and I’ve read reports from others online who observed a similar result with older bottles (I was using a 10 month old bottle and it wasn’t working so I went searching).
When that happens it can be offset by just adding a bit more, though, to up the concentration. You may need to double that dose in 6 months or just buy a new bottle (fyi).