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.
She comes and goes. Each morning it is her face that replaces the darkness (15-16)
Now that the mirror is a lake, we actually see one of those faces that the mirror referred to in the first stanza. It seems that this woman's face has replaced the pink wall in prominence now that the mirror is a lake, though it's not quite a part of the lake's heart. This quote shows that time has passed in this poem, as the woman comes and goes from the lake and returns each morning – but we don't know how long she has been coming.
In me she has drowned a young girl, and in me an old woman Rises toward her day after day, like a terrible fish (17-18)
The entire poem has been building up to this image: the woman seeing time pass in her reflection in the lake. Her youth is dying, and her old age is rising, day after day. Time will pass for this woman, whether she likes it or not, and her reflection will show it.