How long does it take until a tank is considered "mature?"

I'm not the board but here is my .02 cent worth (stold your line Beaslbob) In about 1 year, take it that the tank is complete with LR, LS (optional) and bio material in place.

It varies, depending on setup, bioload, amount of LR, etc.  However, as mentioned a typical rule of thumb is about a year once you stop fooling with it (changing equipment, stocking level, etc).

Tanks are like children, and go thru many changes along the way (algae blooms, chemistry shifts, etc).  Pretty much, a well-managed, mature tank can handle most upsets easily...some may "burp" but usually that's about it.

Maturity = stability.

+ one especially the part about fooling with it.

At least a year old and it needs to be extremely stable. Even at a year old, there are a lot of tanks out there that I wouldn't consider "mature".  A lot has to do with how much knowledge the hobbyist that is maintaining the tank actually has.  My 180 setup is technically only 6 months old, but the sump, rock and most of the sand and coral came out of a previously setup tank. Since the sump is the same, all I really ever did was increase water volume when I moved to this setup, so when people go, "Wow, that's only 6 months old," it's really not an accurate image. A six month old tank, usually doesn't look or act like mine is.

I say if you start from scratch and stock the tank within the first three months then you're looking around 9 months to a year, depending on the hobbyists abilities. Some tanks can't keep some SPS corals alive until the tank is at least 9 months old... after that, they flourish. No one can really figure out why yet.