This mostly depends on your climbing frequency and the conditions where you climb. On average, a 1-2 x per week climber can probably get by with 5oz or less in a month. 3-4x per week needs 5oz. and 5-7x per week needs 10oz. But be prepared - as soon as your deadbeat, greedy friends try it, their hands are going to be in your chalk bag constantly, so you might need more than you think!