What do you mean by saying "lifespan of a software product and a website"?

If a software and/or a website is maintained it usually keeps on evolving by the request of its users or by the ideas of the developers.

Take a product like Microsoft Word, it's first version came out in 1983 and it still exist today after going through the evolution of DOS, Windows and Mac operating system. This makes Word 24 years old which is quite remarkable.

There are also various web sites that exists as far back as the early 90's.

So it seems it would be hard to calculate the lifespan of a product. Perhaps you should try to find a formula which makes a product successful and long lasting.