How long does a DAC take to burn in?

As others have said it really depends on the unit and design and really more than anything the capacitors used. Also keep in mind as most Dacs get left on all the time it can help breaking in caps to cycle the until off and on from time to time.

It's a combination of initial break in, and warm up. Unless the manual says otherwise, leave it on 24/7.

There are burn-in CDs that can help for example :

https://www.telluriumq.com/system-disc/

Most high end equipment well take between 500-600 hrs to break in, I leave a new piece of equipment running straight thru to 600 hrs 24/7. Usually between 3-400 hrs you will start to hear what the piece will start to sound like but the magic starts at 500 to 600 and up to 1000 hrs for some pieces like Accuphase,Arye and others.

Not all, but some high end equipment fully breaks in within 2 to 8 weeks of you selling it.