If you're keeping a tally of the length of time I stood in lines during my trip, the Musee d'Orsay took about 35 minutes (I went on a Thursday, when it is open later) and the Catacombs, which I will discuss next, took about an hour and a half. It seems bizarre that something like this would become a tourist attraction, with its own gift shop at the exit no less, but this is yet another example of the darkness and light that Paris is famous for.
The catacombs, for those unfamiliar, are a network of underground passages wherein the remains of over 6 million Parisians have their final resting place exposed to the camera flashes and airborne bodily functions of over 6 million tourists a year. Human bones aren't something people see every day and so it is somewhat fascinating; both a reminder of one's mortality and a great advertisement for cremation.
The walkabout starts out with a very long spiral staircase descent and then continues with long walks down endless passages. It was a great workout to work off all the flan I had been consuming. Recent storms during my trip made the pathways very slippery though and partially ruined the shoes I was wearing. Being literally SURROUNDED BY DEATH made this but a small inconvenience. There were bones everywhere! There were also tour groups everywhere. It is always very easy to pick up little factoids - in English, Mandarin, Klingon, etc. - about whatever you are visiting in Paris just by walking through. The most interesting thing I overhead while in the catacombs was word of thousands of university students using them as a party site for a rave in the 1960s. Can't imagine how this would be an enjoyable evening...not just for obvious reasons, but also with how cramped the space is. Mind you, I am a pretty boring person. Rest in peace.