This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
3 to 6 months
Placement is usually somewhere along the groove in the nostril where
the cartilage is often thinner. The interior structure of the nostril
should also be examined when deciding placement.
Some piercers perform nostril piercings using a needle one size larger
than the jewelry to be worn (e.g. 18 gauge jewelry, 16 gauge needle)
to allow room for new tissue to form around the inside of the
piercing, but this is not necessary.
Initial jewelry: Captive bead rings in 20 to 14 gauge and 5/16" to
7/16" inside diameter. The ring should not hug the edge of the
nostril. A ring which is too small in diameter will often cause the
piercing to migrate or scar.
Nostril screws in 20 to 16 gauge and 5/16" and 3/8" in length; the
nostril screw should be 1/16" longer than the thickness of the
nostril. Nostril screws are used when the wearer prefers the look of a
stud. Nostril screws have a small semicircle at a right angle to the
stud that holds the jewelry in place without the need for a
clasp. Nostril screws are more difficult to clean than rings.