It's really hard to tell you how much glycerin
your hair needs - as, well, it's your hair.
My needs change depending on the weather - when the air around me is drier, I need less glycerin - too much would cause the glycerin to pull moisture out of my hair, because there isn't an equal amount of moisture in the air. In more humid times, I can use more, because it will suck the moisture out of the air, and into my hair.
Also, the more moisturized my hair is, in general, the less glycerin I need.
I'd suggest experimenting - you'll be able to tell pretty quickly what the 'right' amount is for you and your hair better than we could tell you.