How to you gauge when to leave a planet? If you are interested in just completing your class quest, I have found that you can easily take these quests on one level behind the level indicated in the mission log. For example, if your class quest is designed for level 25, you can easily finish it when you are level 24. You may be able to complete the quest two levels behind, depending on how tough the boss is or how good your gear is.
The class quest is the metric that I use for moving onto a higher level area or different planet. If I am on par with my next class quest level, I tend to skip other quests in the area. Now that I am on my third character, I want to finish the story as soon as I can. I took this approach with my last character since I was mainly interested in PvP. While I did hit 50 faster than my first character, I didn’t have as many companions maxed out on affection, nor did I have a high dark side alignment rating.
If you just want to check out the class story for a character, use the class quest level in the mission log as a guide on when to move on. If you want to increase other areas of your character, such as DS/LS alignment, codex entries, or companion affection, take on as many side quests as you feel comfortable doing.