Newbie gains might be large enough to measure monthly, but gains following the newbie period tend to average out to a meager amount on a monthly basis, maybe 1/16th inch. I’m not too confident that measuring accurately to 1/8th inch is too reliable given how much erection hardness varies. With all that in mind, I think monthly measures are plenty often. Measuring every two or three months would probably be better if you have the willpower to leave the ruler in the drawer.
calfaddict