I would plan for a whole day at Versailles, it is worth it in my opinion. I would get up early and take the train so as to arrive about 60 min before the palace opens, the walk will take 15 minutes to get to the gate and you have to buy your tickets before you get in the long line to get in to the palace, please listen to this! I waited for 1.5 hours my first time there to get to the entrance only to find that you have buy the tickets in a side building before you get in line. If you have a museum pass (Paris Pass) it will get you in without a ticket. The tour of the palace takes 1-1.5 hours, then go into the gardens and plan on spending the next several hours just strolling, have lunch back by the lake were the boats are docked and after lunch go see the Summer Palace and Marie Antoinette's peasant village where she and her ladies in waiting use to spend the day being "ordinary people" in. If you get tired do what we did and take one of the boats out on the lake, it's relaxing and you see the gardens from a different perspective. I love Versailles, and have been there four times and would go again if fate allows me.