DAC break-in...

I quite like capitalism, but with Pareto I would claim it requires a transparent market. These monopolistic competiton tricks by boutique manufacturers claiming differences that do not exist are a scam.
As for DACs, I am usually listening to the internal DAC of a Chromecast Audio, and even on a very revealing system (Quad 2805 speakers) the sound is fine. DAC chips cost a few dollars only, if bought in quantity. Designing their implementation is a piece of cake if you just follow the instructions of the DAC manufacturer. Of course there are DACs that  measure better, but the critical question is if they sound better when observed under controled conitions.
My other DAC is an ODAC.

after my Aesthetix Romulus CDP was unplugged for 6 months, I called Aesthetix and was told one month on standby would bring it back to full performance, plus playing CD's of course...I would call the manufacturer and ask...can't think of a reason the engineers who designed it would mislead you if it really only took a few minutes...

but if break-in occurs while it is on, even if not playing a CD, that changes your whole time frame...

There was a definite period when I first acquired an Ayre Codex that it presented very closed in and two dimensional compared to the loaner that I had demoed in my own system.  I ran Roon radio into it for many hours (days) with and without my preamp powered on, occasionally listening.  The period of time involved until it settled was very consistant with what Ayre predicted. 

...the DAC's I am interested in is notorious for a lengthy break-in.

If you're speaking of a Schiit DAC, it is my understanding that many owners/users claim an extended break in period is required, while the manufacturer does not. The manufacturer does claim that leaving it on 24/7 is best.

I don't particularly believe in break-in, however, certain components such as capacitors do need a little time to "form" correctly and circuits can "heal", but these events happen (IMO) soon after they are initially electrically charged. Electronics that need to operate at a particular temperature is another story and separate (IMO) from "break-in".