Several decades.
The growth of dinosaurs occurred quite rapidly until they reached sexual maturity, after which it slowed down. For example, the Albertosaurus, reaching 9 meters in length and weighing up to 1.7 tons, became an adult at about 16 years old (many skeletons of Albertosaurus of various ages are known).
It is believed that the largest dinosaurs required 40 to 50 years to reach maturity.
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