Hi Badnaam,
Same as you, I am not completely bald either and I've been wearing a hair system for about 10 years. I normally just trim my front hair line and areas where the tape/glue is attached to (traces about the size of tape). I keep my side and back hair around 1" to 2.5" depending on the hairstyle I want to achieve. The remaining areas on the top is about 1/2"-1".