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 2.5oz in a month. 3-4 x per week needs 5oz. and 5-7 x per week needs 7.5oz. 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!