Hypothetical question: If the human race disappeared from the earth today, how long would it take for all traces of civilization to disappear?"

Depends on what you meant by traces. The spacecraft from Apollo program will be "fairly" intact for quite a long time. There are also other spacecraft, such as Voyager which will last millions of years (won't work, but would qualify as "traces"). Also our activity with nuclear material would leave a significant trace that would be readable for at least 100 thousand years.

"How long would it take our infrastructure to crumble to dust?"

Well, not very long. Dams need to be maintained, without dams, you would have floods which in a couple years could destroy a large part of many cities. Then you have things like forest fires. Then, you have vegetation.
Things need to be maintain, roofs need repair, or water gets in, and then things like termites get a foothold, so within a decade or two houses start falling apart. Big building are probably even more vulnerable- if water is cut off then they might last longer, but things like earthquakes, and general corrosion will weaken them. Generally, towns in deserts will last longer.
Anyways, within a decade or two our infrastructure wouldn't be infrastructure, but evidence could found of it once existing for 1000s of years. In 1000 years it wouldn't really visible- you would have to look in right places, and maybe do some digging.