i transplant about ever month and a half, when the roots start coming out the drainage holes and your plant is obviously too big for the pot then its time to transplant, i think of it like it grows two direction, just as fast up as it does down, so if your plant is over 3x bigger than the pot, it is obviouse to transplant, i havent had any problems letting the roots get root bound, they seem to always untangle themselves, i dont pull at the roots i just take it out of one pot and put it in the next.