To answer your question about how long tapes will last. Data tapes, when kept in a temperature and humidity controlled environmemt will retain their ability to hold data virturally forever. However, as you pointed out, the condition of “bleed through” or “print throught” will gradually erase the data that is on the tape. To prevent this, tapes must be rewound periodically. Most companies having large amounts of data stored on tape will employ a tape librarian whose duties include the rewinding of tapes. Most companies will rewind every few months. If you keep your cassette tapes away from excess humidity and rewind them periodically they should last far longer than you or I will! Of course this assumes good quality tapes that have not been exposed to ultraviolet (which destroys the mylar substrate of the tape) or excess heat. The general rule is that if you are comfortable in the environment, then so are your tapes.
As far as floppy diskettes are concerned, they are basically just round tapes. They also are made of mylar, and the coatings used are basically the same. However, with diskettes you do not have the problem of bleed through because the mylar is much thicker than that used in cassette tapes. At least that is what I have been told. On a diskette, the problem is that the data is stored by aligning the iron oxide particles in the coating either north-south (representing a 1) or south-north (representing a 0). Or vice-versa (I never can remember which is which). Anyway, the north pole of a binary 1 will attract the south pole of an adjacent binary 0 and eventually cancel each other out. The designers take this into consideration when they plan the data density that the diskette will use. As hardware became more efficient and the quality of the coatings improved, data densities have gone up without sacrificing reliability. This means that in the same environment, the diskette will hold data just as long as tape will. A friend of mine has floppy diskettes from an old Ohio Scientific kit computer from 1979 that are still readable after 13 years. I have at the college 8″ diskettes dated from 1972 that are still readible and reusable on a System 34 minicomputer. That’s 20 years!