From tiny newly-spat 1cm fry to maxed-out full grown adult, several years. Not a precise answer, I know.
The time it takes for mbuna's to attain full grown size depends on variables (species, water quality, diet, genetics, tank volume, etc).
Give or take they'll typically attain 'adult size' at about the 2.5-3yr mark (age from birth, not from when you got them). Subsequent growth continues to the limits of the species but growth rate is reduced.
Sexual maturity occurs sooner. I've had female juvies still in a grow out tank mouthbrood fry only 10 months after being born themselves.