I happened on this great page on the Musée d’Orsay site. It tells you the best days to go and even gives a link to a page that gives current wait time. The Musée d’Orsay also has an app.
When we went last summer, my son and I were very happy to discover no line at all on a summer afternoon. The chart below gives sample “rush hour” timing.
Remember, kids under 18 enter free to most museums including the Musée d’Orsay, Centre Pompidou and the Louvre. Tickets are also free for certain students under 25 as well. The website for the Centre Pompidou has a similar chart for busy times.
Source: Musée d’Orsay: Rush hours