This mostly depends on the type of chalk, how often you use it, and the conditions in which you’re using it.
For example, when using loose chalk, you’ll want 10 oz. per month if you’re going hard in the gym 5-7 times a week. If you’re only going once or twice a week or not using much chalk, start with 5 oz. or less.
For liquid chalk, a single 75ml bottle of Secret Stuff® will last about 100 uses.