Veterans Day Memorial in the U.S For the ones who know about it:
How many more times will the Veterans Day Memorial stones' holes "align" perfectly?
Some info from this site: 

At precisely 11:11 a.m. each Veterans Day (Nov. 11), the sun's rays pass through the ellipses of the five Armed Services pillars to form a perfect solar spotlight over a mosaic of The Great Seal of the United States.

I know that there are probably several factors which could affect it (like tectonic motion or earth's orbit maybe), but if the designers hadn't been somewhat sure about its permanence, then I guess they wouldn't have used it for something of such importance to the U.S. So my question is essentially, for how long have they made sure that it works (from a physicist's standpoint)?
 A: The answer to your question can be found in the description of the engineering of the monument
You can read the whole story at that link; I will just quote the most pertinent statement:

Using the statistical mean of the 100-year data, the altitude and azimuth angles for the structure were adjusted to provide time/error fluctuation of plus or minus 12 time seconds from the International Atomic Time mark of 11:11:11 a.m. That small time difference allows for additional compensation of the variations that you mention. We also checked the variance 500 years out, and if the structure is still standing, it will work.

I added the bold in the quote above. So the answer to the exact question you posed is: "They made sure it works for 500 years" - but probably longer.
Incidentally, you can check this yourself by putting numbers through the NOAA online calculator to see how much the azimuth and elevation change from year to year. The answer is: very little.
Here is a snapshot of the calculation for yesterday:

