At a minimum 1/2 a day due to the distance from Paris (30 minutes by train).
You may want to do this: Versailles for the day / afternoon, get off at Port Alama Train Station and take the Bateaux Mouches boat ride during sunset / evening (only €10. Nice way to relax after a long day plus it's on the way back from Versailles: http://www.bateaux-mouches.fr
It's unfortunate Versailles doesn't have any re-enactors or very much English descriptions. I found the English (UK) museums / historical sites have better presentation and are less "stuffy".