How often do guards change at the tomb of the unknown soldier?

every 30min. in the summer every hour in the winter and every 2 hrs. when the cemetery is closed

I'm pretty sure it is every 30 minute; the soldier has a certain amount of steps, positions, and gestures that he/she HAS to perform within each interval before another soldier takes their place.

Did you know that each soldier has to perform each interval perfectly; they each have 3 times to mess up before they are permanently restricted from guarding the grave site.