Normally coral reefs take many millennia, but fast colonization time scales coral can proliferate pretty fast. Soft corals are the fastest growing. It typically takes coral 25 to 75 years to reach sexual maturity.
Montipora capricornis (Leaf Plate) can also grow pretty quick....All in all it takes 1000 years to get a mature reef unrecognizable from natural ones 10,000+ years. Since corals build atop existing skeletons it's hard to gauge their age.