Cresteds just like snakes grow at different rates, but how big is he? You call him he, but do you know he is a he like does he have a noticeable hemipenal bulge (will look like testicles)? If his bulge is apparent I would say over 6 months. To gauge humidity take notice of the time your cage takes to air out between mistings. It is ideal for water droplets to remain for 3-5 hours, be dry 4-6 hours, and then remisted at night because they do need dry periods to aid proper shedding and avoid respiratory infections.