Wine from Juice

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.