But how long do we want them to last?

For cars, perhaps at least 30 years, or at least 200,000 miles.

For phones, at least 4 years.

For laptops, at least 6 years.

For digital cameras, at least 3 years.

For films, great films will be good for 50 years.

For philosophers, at least 100 years is sufficient.

For human life, at least 70 years is sufficient (according to Nietzsche). I would prefer 120 years, but 92 years will also do (when my grandfather passed).