I think there are two questions embedded in yours - one obvious, one hiding.
The hiding question is "how long until it is obsolete and [possibly to you] worthless".
The surface question is "how long will it operate reliably".
If you buy a quality DAC at the top of the food chain ( in its range) the answer is "quite a long time". While i have new/newer DACs and prototypes I’m working on (excellent, but have, uh, issues at this early stage, kinda like 2 year olds), i also have two very old DACs and both are useful.
One’s a circa 1999 (?) MSB Gold Nelson, with a bunch of modifications I made that improved it significantly. It works great, sounds "good" but not state of the art, and has a big flaw (to be corrected) - S/PDIF input only - no USB or network.
I also have an even older one; a Theta DS-PRO II from what, 1989? 2nd Gen of the DAC that started DACs. It was also S/PDIF only (duh, i came out when that interface arrived) and continued to sound pretty good. Again the bog caveat was that i needed a USB or network --> S/PDIF converter and most suck. There are a very few pretty good ones, like Mike’s Piece of Schiit, but it doesn’t play well with the Theta - they get in synch fights. Ironic since Mike designed both, albeit 40 years apart.
So i built and inserted a USB --> isolation --> re-clock --> clean power stage before the S/PDIG and stuffed it inside. Now it soudns much BETTER than it did, presumably with less jitter and ground noise.
I realize this is not a path most people can take, and normally i would not waste the time, but the DS Pro is such a piece of audio engineering history (look it up, very innovative and cool) that i just had to save it.
So the moral of the story is that within reason, they can both last and continue to be useful and good sounding - but tech may move on.
G