Assuming that a floppy disc is stored in a sensible place and is not chewed by the dog etc, how long will the data that is stored on it last? I heard a disturbing rumour that they last only about three years.
Basically they will last forever if kept in a dry non-humid place. The worst you can do is to leave them close to any form of magnetic influence but even then it would have to be extremely powerful to make complete garbage of the data that is stored. Best advice is to back it up on another disc and clearly label them both.