"The furthest back in time that we are currently actively seeing is
13,7 _ 0,15 billion years -- 379,000 years
The 13.7 billion years is the currently measured time to the Big Bang and 379,000 years is the number of years after the Big Bang when the universe cooled off enough to become transparent. The (mostly visible light) photons from the hot plasma that filled the universe at that time have been traveling since then and have now been red-shifted down into the microwave range. This is the Cosmic Microwave Background radiation that has been very accurately measured by the WMAP satellite.
We will not be able to see back further in time (to before 379,000 years after the Big Bang) with photons since the universe was opaque to photons before that time.
However, if we are ever able to use neutrino telescopes to measure very low energy neutrinos (which is probably impossible), then we would be able to see back to a few minutes after the Big Bang. Finally, it is also theoretically possible to see back to roughly seconds after the Big Bang if we could measure the possible gravitational waves that could have been generated at the end of the inflationary period of the Big Bang at that time."
If there was a mirror a million light years away and I looked at via telescope, how far back would I see in the past?