You should know when it is silent enough. If you are in doubt... it's not! Also think in terms of vibrations. If you feel vibrations from the landing traveling up your body, it's too high or you are too fatigued. If you can absorb a 30" box, it should feel pretty much like a 20" box. I've found that it's very abrupt for me... a certain box will be effortless and easy to stick the landing, but a couple inches higher and I can definitly feel it. It could just be me though.
As far as how you land, you have a lot of different options. Obviously the deeper the squat you land in, the higher the force you will be able to absorb. Ultimately you want to develop posterior chain stiffness, which means a very quick absorbtion phase. However, you have to have force absorbtion ability first, so you may need to land in a full squat. You also might want to try stiff-legged landings if you have a plantar flexion weakness- which is a very good possibility. I think it's very individual.