I am often asked to predict how long of an evaluation period it will take to decide if a new product stays or go.
If we ignore burn-in (something not easy to do since some equipment requires a great deal of it while other products none at all) then much depends on a number of factors including the resolving power of your system, the kind of music you're most familiar with, and your ability to discern sonic attributes.
It's clear that resolving systems make it much easier to hear differences. That's what makes them resolving. That said, is the difference you hear better, worse, or simply different? Here we need to go back to basics and reference what a real instrument sounds like—veracity settles the argument.
…unless we worry about point number 2. If your favorite music is electronic, amplified, or somehow artificial (as opposed to an acoustic instrument), then the playing field shifts. In this case, one's best option is likely the good old human voice.
Lastly, how confident are you in your skills as a listener? Do you make quick snap decisions or agonize over minute details for long periods of time?
My go to answer is 1 week. Spend a week living with the new kit and if at the end of that 7 day period you either don't know or cannot for whatever reason decide, probably time to send it back.
But, your mileage may vary.
