This depends upon whether you are using free-standing studio monitors or mounted, flush units.
For free-standing equipment that is not tilted down, the ideal height is at least 47 inches from the floor with your tweeters aimed directly at your ears for the best listening quality.
If you want to tilt your speakers down, go for about a 15 degree tilt but no more than that. The frequency changes if you tilt more than this.
For main studio monitors that are flush-mounted, recommended height is about 56 inches off the ground in order to give you the best sound quality whether sitting or standing.
In the event you are going for the surround sound effect, mount speakers about 2 feet above seated ear height for best listening vibe.