Prior to burn in, I generally hear what I'd describe as an uptight sound, lack of transparency, also, excessive attack , attenuated decay. I'm not going to generalize about tonality, timbre. For sure you'll know when they reach burned in state, often the change comes on in what seems an instant. Suddenly you'll hear an opening up, could also be called blooming of sound. I'm sure the burn in process is slow and ongoing, but almost without exception, the last bit happens in just an instant. Many say 100 hours is usual for the process, I hate to say any particular hours, expectation bias sets in with a set amount of hours. I'd say anywhere between 75 and 200 hours should get you most of the way there.
I usually don't even bother listening anymore prior to 75 hours or so, why suffer the burn in if you don't have to. You may be tempted to come to conclusions prior to burn in if you have to suffer through disappointing sound for too long. If new to cap swaps you may want to hear at least occasionally during burn in to evaluate before and after burn in sound.
