If you are underwatering they will be wilting before you water them. As long as they are growing fine, you are good.
For the OP, it depends on how big you want them to get. You can figure on them getting about 2-3x the height they were when you started flowering. If you start flowering at 18 inches they could end up being around 4 1/2 ft tall. Indica dominant plants will be towards the shorter end, sativa dominant closer to the taller end of the scale.