how long does it take a bonsai tree to mature,

I hope this is not a trick question from a troll. It depends a lot on what you
mean by mature. If you mean how long is it from the time you plant the seed,
cutting, or starter plant to the time it is ready to put in your local bonsai
show, the answer is anywhere from two to twenty years.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)