How long does a DAC take to burn in?

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.

^^^
funny

A couple months ago I was curious so I asked the "new" ARC by email - how long does it take to burn in your DAC ? ARC Customer Service responded back "look in the manual". Nothing in the manual about burning in.
Make what you want with their comments.

Prior to this, quite a while ago when I first got the DAC, and was still able to communicate directly with staff at ARC - I was given an hours estimate.

Cheers