Well, when they reach sexual maturity. For example, convicts can be 3 months old and attempt to breed. Oscars take 12-15 months to be able to breed, in good conditions.
Green Terror grow very slow so you may have to wait a while for them to get big enough. I would say at about 5-7 inches the reach the point where it could be possible to get a breeding pair.
As stated before, you asked a broad question. But i hope this helps.
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