Hm, depends on your hair length, obviously. I have to use massive loads and do it twice to get better colour. It's best to mix it with orange juice or something else that's acidic, some people use red wine or vinegar. Orange juice is just what I prefer. It's a kitchen witchery thing, getting the right consistency is a trial-and-error business:P... somewhere between toothpaste and yogurt is good. And oh, heating the juice improves the results massively, as long as you don't boil it. When the gloop is still warm, mash it into your hair (I've given up on the applying with the comb business) so you get big floppy dreads/tentacles/whatever, as long as it's spread all over, then wrap that in a clingfilm turban around your head (or a shower cap or a taped-up plastic bag if necessary). Then let that set for 45 minutes, with your head somewhere fairly warm. Wiggling a hair-dryer around it all the time to keep it hot or something (I just sit in a heated sauna but for obvious reasons I suspect whether most people have the chance to do so). Rinse and repeat if necessary.

Basically heat and acidity and patience are of the good. The acids help release the tannins from the henna by oxidisation and stick better.

Hope that helps!:)