My apologies, I didn't take the time to read through all the replies, but here is what we did:
l'Orangerie: less than an hour (mostly there to see the waterlilies)
Louvre: a few hours, then lunch break, then back for a few more hours (only scratched the surface...you could spend days there. I made a list of the most notable pieces and which room they were in (listed on the Louvre's website), then when I got there I circled the rooms on the free map so that we could visit them in sequence. We took advantage of the one night per week that they are open late, and that helped a lot since we are not early risers.
Versailles: Again, we are not early risers, but we spent the better part of the day there, including strolling the grounds (which are quite large and include additional points of interest).
We also visited the Musee D'Orsay, but unfortunately got there too close to closing, so it's hard to say how long to spend...probably a couple hours.
The museum pass is great, as it is good value (depending on how many things you are trying to see) and allows you to skip the line. It also allows you the freedom to pop into museums just for a short time if you'd like rather than feeling obligated to stay for hours because you paid full admission. We popped into the Rodin Museum for 45 minutes or so just because we could and enjoyed seeing what we might not have otherwise stopped to see. One important thing to keep in mind is that (unlike the US, or at least where I live), the museums are not open late on most days. Many are only open till 5 or 6, so that will limit things if you are trying to do multiple in one day. Also, it is important to note that there is a time of "final admission", meaning the time that they'll stop letting people enter. This is usually 45 minutes or an hour before the actual museum closing time, and is strictly enforced - we arrived at the Orangerie 30-40 minutes before closing on our first attempt, just wanting to take a peek at the waterlilies with our limited time, and they wouldn't budge on letting us in as it was past the time of their final admission. I personally would not do more than one museum in a day (unless they are some of the smaller ones), but that also depends on what type of person you are/how quickly you get burnt out in a museum setting.
I don't think I have a feeling of regretting rushing through any of the museums. What I wish most is that we'd had more time to slow down and just enjoy the city. It was our first trip, so there was tons to see, but that resulted in us being on the go most of the time. Wish we would have had more time to pop into the markets, sit at a cafe, etc. Be sure to know exactly what you want to see and where it is located so you don't end up backtracking to the same neighborhoods (if you are limited on time). Hope you have a great trip...Paris is my favorite place!
