The amount of time a miner is economically viable for should be increasing as "Moore's Law" for ASICs slows down. Just the same laptops are usable for more years nowadays than they were in the early 2000's.
Also - if you have free electricity, which is a thing in certain settings, where prices can and do go negative, mining hardware will be viable for "a long time".
Also - if you have free electricity, which is a thing in certain settings, where prices can and do go negative, mining hardware will be viable for "a long time".