How long do you or should you keep your gear.

I've had my Quicksilver Mid Monos since 2015 and while they are being replaced, I have no intention of selling them.  Same thing for my cj Classic 2se, it served me well since 2014 and I just replaced it with a Zesto Leto.   I can say will certainty it will be the heart of my system for a while, its that good.    

I've had my Magnum Dynalab MD 102 for about 10 years, no intention of selling ....

DACs ?   Disc spinners? They come and go a little more often, I almost view anything that spins a disc as a consumable.  They die, get out dated.

DACs can be viable for a while  but are forced into obsolescence  by new codes etc...  I spend up to the point of diminishing returns , relative to the rest of my system on those.

I've actually had good luck with tubes...QS has been super reliable.   I'm actually going to put the CJ pre and QS monos in a second system.

I auditioned my last two sets of speakers on PL integrated amps and thought they sounded quite good.