Even though it was built in the 17th century by Louis XIV as a hospital in order to house invalid soldiers, the memory of Louis XIV is not very present inside the building nowadays... as Napoleon stole the show!
In his will, Napoleon had expressed the wish to be buried "on the banks of the Seine, in the midst of the French people [whom he] loved so much". So the choice of the Invalides, a building dedicated to soldiers, became obvious as his burial site when the French authorities finally accepted to bring his body back from the Saint Helena Island in 1840 (Napoleon died in 1821). That's why the Royal Chapel, originally built for Louis the 14th by Jules-Hardouin Mansart, now holds the huge sarcophagus containing the coffins of Napoleon inside the crypt which was inaugurated in 1861. If we visit only the chapel, it takes between 30 and 45 mn (we can easily combine this visit with the Rodin museum which is next door) but, if we want to visit the Army museum (old armors, helmets, uniforms, flags, paintings and a lot of WWI and WWII memorabilia), you can easily add a couple hours. The entrance fee includes automatically both the chapel/tomb and the Army museum. My advice, if you use my services and you want to save money, is that we finish the tour at the tomb and you see the rest on your own (I'll give you directions, of course).