Pick outside leaves as early as three inches long but before leaves grow to 10 inches long. Older leaves will have an earthy flavor. Harvest chard on a cut-and-come-again schedule; remove a few outside leaves at a time. If you harvest the whole plant, cut it back to about 3 inches above the soil, and it will grow back.
For more information on growing Swiss Chard, check out this great guide from Harvest To Table: