If the lemon juice was properly stored in the refrigerator the lemon juice should last from 12 to 18 months.
But if the lemon juice wasn't properly stored in a sealed container then the lemon juice can go bad much sooner even when refrigerated.
You can freeze lemon juice and keep it longer than 18 months if you want too.
You can tell when Lemon Juice has gone bad by it's appearance, odor and taste.
If the lemon juice is dull or has become darker, throw it out.
Do the same if it's odor or taste has changed noticeably.