How long does invisible lemon ink last?
I think the hidden message won't be lost, it will become visible instead.

I don't have any practical experiences after what time the message will become visible, but I'm reasonably sure it depends on the type of paper you used and under which conditions the paper is stored.

Lemon juice as "invisible ink" works by oxidizing the paper, which accelerates it's ageing. Applying heat further accelerates the chemical reaction. 

If you put a blank sheet of paper on a hot plate, it will eventually become brown. The parts where you applied lemon juice discolor much faster, which makes the secret message visible. It's not the lemon juice that gets brown, it's the paper itself that does.

If you don't actively accelerate the reaction by applying heat, the lemon juice will react with the paper much slower. Direct sunlight and a hot environment would probably make the message visible within a year. In a cool and dark storage the message could stay hidden for several years.