The splendid palace and gardens of the Sun King, Louis XIV, require several hours to explore at a minimum (travel to/from central Paris on RER takes a bit over half an hour, but budget for about two hours in total on door-to-door round-trip). You may want to budget a full day, since the attractions are aplenty.
Versailles was the first royal palace that we visited on our foreign travels, and it made quite an impression. It is worth mentioning that after having seen scores of palaces across Western Europe, the opulence of royal apartments does not ignite quite the same reaction in us. The Hall of Mirrors in Versailles’ Grand Apartments ♥♥♥, though, is still among the most ostentatiously magnificent, and would be worth the visit all by itself. Other rooms have something to boast as well, although they are always crowded to the seams.
The excellent audioguide will also provide narration for Dauphin’s Apartments ♥, which draw only a portion of the visitors.
If you visit Versailles on a passeport, you will also have access to a smaller palace called Grand Trianon ♥, the estate of Marie-Antoinette ♥♥ that contains Petit Trianon and several other pavilions and locations, and, of course, the vast grounds ♥♥ with a boating lake and fountains.
The fountains, unfortunately, are only turned on twice a day for an hour and a half on Saturdays and Sundays in summer. They bring the park to life, but are a fairly sore sight when not in operation. I guess there must be a good reason why they are not on all the time during opening hours, but the fact is, you need to brave greater crowds on a weekend and structure your visit in a specific way if you want to see the fountains working. We finally did it on our most recent trip, and certainly enjoyed the sparkle of the functioning fountains – some of which are truly spectacular – but it is probably not worth it on balance to specifically aim for that. (The presentation of the fountains is called Eaux Musicales, but is nothing like the musical show that one can see in Barcelona or Las Vegas. The fountains are on, some classical music flows from the speakers. The “grand finale” is entirely disappointing – just an extra round of one fountain operating while the same music plays on.)
The gardens are worth exploring, but I don’t deem them anything exceptional, especially when the fountains are off. You can hire a golf cart with an audio narration, which seemingly is worth doing in advance if you are interested.
In fact, buying tickets in advance will save you a lot of waiting in line any time of the year (you may still need to stay in line for security checks – which can take up to an hour in the late mornings – but at least you will not have to spend time in a longer line at the ticket office). If you are coming from Paris and plan to see as much of the estate as possible, then certainly buy a combined ticket at an RER station at Orsay or Invalides, which includes round-trip travel and passeport for full access. Cost per adult is €27.50. Children only need tickets for travel, the palace entry is free.
There are several café shacks on the Versailles grounds, selling sandwiches, panini and the like.