Less than you think. When you first start using climbing chalk, you may think that you need a thick base layer for best results. The truth is that too much chalk leads to clumping and reduced friction, which results in an insecure grip that's likely to slip. Ideally, you want enough chalk to keep your hands dry, but no more. An excellent way to ensure that you've got the right amount of chalk is to dip your hand into the chalk bag, spread the chalk from your palm to your fingertips and then shake off any excess back into the bag. You can also tap your hand against something to remove any loose particles. A chalk ball or sock is also a great way to powder your hands uniformly with chalk. Since different climbers produce moisture at different rates, some people will need to reapply their climbing chalk more often than others. It's vital to only reapply when necessary. You should only consider reapplying after you've completely removed the previous layer of chalk. Before you grab chalk from your bag, rub your hands together to gather any leftover chalk from your palm onto your fingertips. You'll quickly develop a sense of how much chalk to use if you are climbing on a regular basis. The more you climb, the better you will develop your feel for how much is too much or too little in relation to your current situation and grip strength.