"Tepe" wrote:

How long should you keep the wine in the primary?

What kind of wine?

The simplest answer is: until the fermentation slows down to the extent that
it isn't producing sufficient CO2 to allow all that surface-area exposure.
This can vary from 3 days to weeks depending on the temperature of the
fermentation, whether you are fermenting juice or "on the skins" and many
other factors too numerous to mention.

Red wines can remain in the primary longer than whites, because the tannin
acts as an anti-oxidant.

My general rule (others will have differing opinions) is to leave it in the
primary until the cap drops (if fermenting on the skins) or until there is
no more surface foam (if fermenting juice). However, one might wish to press
sooner than that, at which time it would make more sense to move the pressed
juice to a secondary than a primary.