Lanefan
Victoria Rules
Indeed, to the bolded. My answer is simply "as long as it takes".I think a side quest should not be more than one or two sessions long - but ultimately the players decides how long they want to engage with it or the setting. Not me.
Sometimes what starts out as a mere side quest in the DM's mind becomes the main quest in that of the players/PCs, with the previous main quest(s) or story(ies) cast aside either temporarily or permanently.