When roots, followed by small leaves, form at the base of the cuttings, they're ready to be transplanted to a small container with potting mix.
Note: African violet leaves can also be rooted from leaf stems placed in water. Cut off a few young, full-grown leaf stems in late winter to midsummer. Fill a glass jar with water. Cover the top with aluminum foil with several holes punched in it to hold the leaves erect and out of the water; leave one hole for adding water. Keep cuttings out of direct sunlight, and make sure the water level stays constant. Look for a developed root system of 1 inch or so. Don't let the plant stay in the glass long enough to develop "water roots," which won't grow well in soil.