How long does a DAC take to burn in?

Did you add power conditioning, or room acoustics?

I often forget that I've changed one or the other of those.

The general thought is 100 hours.  However, I have seen certain components and metals that may take well over 200 hours to burn in.  Rhodium takes an extremely long time to burn in and also certain electrolytic capacitors take long hours.  It entirely depends on what components your equipment uses and your definition of what "burned in" sounds like.  Some people have higher thresholds than others.

There is no benchmark, it varies by component. In detail, what I have personally experienced is a very wide break-in period variance with no empirical explanation other than an anecdotal summation that the better the build quality, the longer the break-in period. Again, this is only my personal observation .... your experiences may indeed be substantially different.

For example,

(1) My HT electronics (Bluray spinner, AV kit, ~ $6000) break-in period took about ~ 100 hours +/- break-in time, with arguably and ill-defined nominal changes after that initial break-in period.

(2) In contrast, my 2-channel "A" system electronics (digital source and integrated amp ~$25, 000) with a much higher build quality and resulting pricepoint strata, took at least near- 400 hours initial break-in time to reach a certain plateau where any further changes curves now flattened out.

There were some more subtle further slower evolution changes over the next few hundred hours of use.

Note: The break-in period audio performance improvement curve was not a pure straight-line linear one , but rather a series of incremental non-predictable ad-hoc steps.

Without prejudice to any the above , the new audio performance that appears "warmer", and "cleaner" as you highlighted , may actually be a result of the following rather than any enhanced break-in period:

(a) it has a markedly different sonic signature of the new gear brand itself, that you personally (emphasis added) prefer, in conjunction with
(b) a more pleasing audio performance in your bespoke system ( again, emphasis added...) provided by a a "better" system synergy now overall. 

How do you know when it’s broken in? Things change in 100 hours of playing music. Either intentionally or unintentionally so how can you keep track of the sound?

My Benchmark DAC1 was bright sounding at first, then a little bit muffled and then open, smooth and more extended.  It took about 100 hours at least to this point.  There were still very small changes after that, I'm sure.  My class D amp went to about the same changes but it was very slow - about 4x that (400 hrs).

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.

^^^
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

Over the years I've had all sorts of pieces of equipment where no break in was specified by the manufacturer.
But my personal experience is that most things DO seem to improve over a few hundred hours of use.  I'm not sure about things like cabling, but certainly electronic components and speakers (IMHO) do tend to improve with age.

My Ayre Codex DAC took about 400-500 hours to breakin.  I'm probably at 800-900 hours now and they sound fantastic at his point. Very happy with the sound. 

100 hours ! Industry unspoken standard! No true set time! I call it " till it pops " Reproduction seperates and stages! I.e.: life like sound!

I think only speaker drivers open up or burn in...the rest is probably you getting used to the sound.

Nitewulf has very good point and is probably on the right track in my experience.  The mind is very powerful and automatically starts filling in gaps and frequency observations while listening to familiar music.