This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
4 to 6 weeks
The size of the inner labia varies greatly. The labia must be at least
1/8" thick to safely support the jewelry. Piercings should not be made
through the thin folds that are sometimes present. If the inner labia
protrude from the outer labia, the piercings should be placed so that
the rings are not pulled against the edges of the outer labia.
Inner labia piercings can be stretched easily.
Initial jewelry: Captive bead rings in 14 to 10 gauge and 7/16" to
5/8" in diameter; the diameter should be at least 1/16" wider than the
distance between the edge of the labia and the piercing. Circular
barbells may have a tendency to unscrew with friction from walking.
A thicker gauge should be considered if the wearer enjoys pressure or
friction during sex. A thicker gauge will be more comfortable and less
likely to pinch or tear.