Two questions:
1) How long do they stand before the guards change?
2) What started this tradition of guarding the Tomb?
1)They change every 30 minutes.
2)They’ve been guarding the Tomb continuously since 1947.
This can be found online...I don’t remember exactly the answers...
https://en.wikipedia.org/wiki/Tomb_of_the_Unknowns
Answers to your questions.
Very moving to see this in DC. Nice view of DC from there as well.