How long can a mattress and couch last?
Probably 20 years if you took care of it.
despite what the stores selling mattresses would have you believe, mine has lasted for 24 years now and is still going strong. It's just the mattress -- I have a platform bed, so there's no box spring to weaken and break down, leading to breakdown of the mattress. My living room sofa is 35+ years old. We did have to have it recovered [and that was expensive] ... but quality goods last and last if properly cared for. The mattress came from Montgomery Ward and was top of the line when we bought it in 1994. Similarly, my sofa was something like $1100 when I bought it about 1982 -- individually hand tied springs, goose down cushions with springs inside the ones on the seats, and oversize, solid wood frame.
Depends on usage. A piece of furniture with kids jumping on it might not last one month. With gentle usage, they could last for many years.
Depends on whether you use it or not
Depends on what it's made of, if it's used a great deal, and if it's kept clean. A single person's couch used only for company is going to last longer than a family's couch with kids and dogs/cats lying on it or jumping up and down on it.