How far in advance do I need to purchase tickets?

When we went to the Louvre in the summer of 2008 we took Rick's advice and bought our tickets the same day at the underground entrance. If you know for sure what days you are going then go ahead and buy the tickets online. I'm not sure how far in advance the release the tickets. Other posters might be able to help. Have fun.

To refine your expectations, you can anticipate a line at the entrance security checks, some times quick and sometimes not, but unavoidable.
The "underground" entrance is part of a modern shopping mall, sort of incongruous for such an august old institution. Wikipedia says: "The shopping mall is located at 99 Rue de Rivoli in the 1st arrondissement. The mall is located near the Tuileries Gardens, the Comédie-Française, the Musée d'Orsay and the Louvre. The nearest metro stop is Palais Royal-Musée du Louvre (Lines 1 and 7)."
The main entrance in the famous glass pyramid is downstairs too and much busier.

Hi Jackie, congratulations on your marriage. A honeymoon in France sounds wonderful! You can purchase advance tickets in Paris at FNAC. The 2-day PMP is $48. Versailles and the Louvre total $33 so if you want to pop into another museum or two (Orangerie, Orsay, Rodin, Napoleon’s Tomb, Cluny are all good choices!); it may be worthwhile. Have fun!

As people have said the long queues at the Louvre and Versailles are to get through the security checkpoint, not to buy tickets. I haven't been to Versailles recently, but I was at the Louvre in September and there are very large numbers of electronic ticket machines, so it won't take long.

Buying ahead won't help at Versailles. The security line is for everyone and blocks long. The only way to avoid it is to arrive a half hour or more before opening and hope to be near the front or to book a King's apartment tour which uses a different entrance. You should have a ticket ahead, but it doesn't let you skip the security line or use a special line.

The Louvre is just not an issue. You can buy a ticket ahead and use the designated line at the Pyramid, but we usually just use the Carrousel entrance and buy tickets from machines inside and have never had a significant wait in a couple of dozen visits. You might have long lines there in July, but not in April.

The one you need to book ahead is the Eiffel Tower.

We took some friends visiting from the U.S. to Versailles shortly after New Years' Day, and purchasing tickets ahead of time certainly helped -- probably shaved 30 minutes off of our total entrance time, as we bypassed the ticket line, and got directly in the line for security, which took about 10 minutes to go through. . I purchased them online about a week before our visit, which was around 10 AM the first week of January. The ticket line was longer than I expected for that time of year. Lots of east Asian visitors rolling in on tour buses.

You don't need tickets for the Eiffel Tower unless you want to take the elevator up. The view from there is underwhelming and is easily surpassed by the view from the top of l'Arc de Triomphe.