I was not sure how many milliseconds to use, but the more important question
here is how toachieve my goal, which is keeping the <div> the full width of
the page. I am using CSS, only I am doing it through JavaScript. I
originally tried using a simple static style="width:100%;", but that sets
the width to the width of the browser window, not the document. I would have
liked to use the following:

document.getElementById('divTitle').style.width='0px';document.getElementById('divTitle').style.width=document.body.offsetWidth+'px';

Both of the statements above are necessary so that the width can be
decreased when the browser is resized to a smaller size. But even this
method would require periodic updates, since this variable changes with
resizing as well. But because the offsetWidth property is non-standard, it
is not supported by all browsers (one of which is FF). The only other
solution I could come up with is the one I posted. However, because the
values of the variables used to calculate the width change when the browser
is resized or the user scrolls horizontally, I needed a way to continuously
update the width. What would you suggest to prevent resizing and/or
scrolling from interfering with the design?

I actually didn't want to use the setInterval function at all, but I could
not find another way. The number probably is a bad choice, I won't deny
that, I simply chose one while I looked for a better solution (assuming that
there is a better one).