I've had several yellow tangs over the years - usually in the 3-4" category, and to be honest it seemed to me like they never grew at all. During the same period(s) tankmates grew alot ..... a hippo tang from 2" to 6+", a emporer angel from 3" to 8-9", foxface from 3" to 7+"...... that sort of thing..... and the tank(s) were a good size (180 to 240gal)...... for me, they just never seem to grow.
That being said, with a specimen as small as yours, I owuld expect it to reach about 3-4" TL in 12-24 months.