For example if you assumed a patio should be good for 20 years
Your example is clearly *******s as no one expects a car to last 50+ years.
Hang on. First you say that someone can assume something can last [20 years] and then you say no-one can expect something to last [50 years].
So do we assume or expect?
BTW how long can someone "expect" their car to last? Does it matter if its a Rolls or Dacia? Does it matter if they keep it in the garage and drive it once a week and service it once a year, or by the coast and thrash it daily and don't service it? Who defines the expectation of a car purchaser? Do you actually know about this stuff or are you just assuming it all?