As early as I find it necassary. Depends on way to many factors to totally answer this Q.

However.. because many areas I hunt are in close proximity to food sources (not Ag.. I'm talking acorn or apple trees.) I prefer a very very early visit to my stand. 1-2 hours before shooting light is often my goal. Especially on heavy pressured land.

I've many times been in stand 2 hours before daylight and within 5 minutes had whitetail feeding below me for hours.. then returning to bed just after shooting light and walking by me unaware.