I always juice once a day - in the morning for the whole day, keep it in the glass bottle in the portable cooler all day, as I'm unable to juice at work.
If I still have any lemon juice left at the end of the day, I just give it to my family members to use in their evening tea.
Don't waste your lemon juice, but don't use it the next day either.
Also, never pre-mix your lemonade in advance (some people pre-mix lemonade for the whole day). Mix it fresh.