I know that QE involves the purchasing of government bonds by the central bank "printing money" but I am curious about what the typical time to maturity of those bonds are. I.e. the typical time before the central bank is faced with the choice of "un-printing" that money (taking it out of the economy) or doing some more QE.
This paper is a few years old. You might want to look at page 6. Briefly, it varies by country and across time.
It changes over time. Any textual summary is probably going to end up out of date.
If the purchases are a large portion of securities outstanding - which is the case - the central bank has no choice but to buy what is available.