wow, I had no idea how slowly the outer planets circled the sun. Pluto, for instance, takes about 250 years to revolve around the sun.
Also, somewhat surprising, is that it is almost impossible to have more than the first 3 planets in alignment. The first three seem to align every ~ 50 years, but that's the best I can get. According to
this page, once every 180 trillion years the planets should be in exact alignment out from the Sun. The closest I could get on the demo here was February 1962, the planets are not in exact alignment, and it's only the closest 7, some planets are on one side of the Sun and some on the other, Neptune and Pluto (then still a planet) are simply defiant.