This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
4 to 8 weeks
The septum piercing is made through the thin layer of tissue between
the alar cartilage (outer) and the quadrangular cartilage (separating
the nostrils). This space of tissue is usually largest towards the tip
of the nose. The jewelry should be no thicker than what the space can
comfortably accommodate. Jewelry that is too thick can painfully pinch
the cartilage. If the space is small but the wearer desires a thicker
gauge, the piercing should be performed at a thinner gauge and later
stretched. Septum piercings can easily be stretched after healing.
If the piercee anticipates wearing spikes and tusks the piercing
should be made low enough to allow the jewelry to fit comfortably
below the nostrils. Please refer to Part 2B of the Piercing FAQ for
descriptions of different styles of septum jewelry.
Initial jewelry: Captive bead rings, circular barbells in 18 to 10
gauge and 3/8" to 5/8" in diameter.
U-shaped septum retainers are available for those who need to hide the
piercing. The spread of the shanks should be adjusted so that the
retainer may be comfortably flipped up into the nose but snug enough
to prevent losing the retainer.
An alternative to buying two pieces of jewelry (ring and retainer) is
to wear a small circular barbell in which the gap between the balls is
wide enough that it can be flipped up into the nose. Wearing a
circular barbell as a retainer is limited by the size of the nose.