They should be 4" to 6" after 12-18 months (of life, not since when you got it). After which growth rate slows to maybe 0.5" to 2" a each following year. Sexual maturity is attained at around 3yrs of age, but full growth (12"+) may take many years to achieve.
To optimize growth rate: feed them twice a day on a high protein diet during the first year of life. Keeping them in very large tanks (125gal+) with very prestine water as soon as they attain 3" will also benefit growth. Refrain from having them stressed or outcompeted by overly aggressive/active fish (such as mbuna).
Also, growth will vary among individual frontosa's (with dominant males typically growing the fastest). Of my group of 5 (which were purchased as 1" juvies two years ago), one is at least 9", one is about 8", two are 7", and the smallest is about 5.5".
i like the curcy stripe ^^