Most of the time I meditate on the opposite wall (6)
This is the first glimpse of the concept of time that we get in this poem. The mirror spends most of its time meditating on, or thinking about, the opposite wall. Time is just included as a word in the phrase "most of the time," but at least we know that for the mirror, time does pass, even if the mirror isn't changed by it.
I have looked at it so long I think it is a part of my heart (7-8)
Here, we see that "most of the time" is actually a significant amount of time. The mirror sounds almost a little mournful at how long it's looked at the wall – long enough for an inanimate object to think another inanimate object is a part of its heart, even though neither object has a heart to begin with!
But it flickers. Faces and darkness separate us over and over (8-9)
This line shows us how the mirror might tell time: from the patterns of flickering faces and darkness that separate it from the opposite pink wall. The phrase "over and over" contributes to what seems like a pretty monotonous life for this mirror.