Age of a planetary body We quote that the Earth is about 4 billion years old. However earth started with billions of cigarette smoke sized particles. What event or size or something do we use to say this is the start of Earth? Can we know how long the formation from dust to current size took?
Radioactive dating works based on the assumption that once a particular rock (in this case, a tiny zircon crystal) has cooled and solidified, the material within it will not be exchanged with material outside it. Since the rock contains trace (known) amounts of radioactive isotopes at its formation, measuring the amount of radioactivity today gives the amount of time since that rock solidified and cooled.

The USGS claims that the age of the zircon crystals is roughly the age of the Earth. They are doing so based on the assumption that the zircon crystals solidified shortly after the Earth was formed. Considering that Earth formed when a diffuse cloud of tiny particles collapsed into a comparatively tiny ball, releasing a bunch of potential energy in doing so, it is reasonable to suspect that Earth, just after it formed, was extremely hot, certainly hot enough to melt zircon at 1900 degrees C. As discussed above, melting something "resets the clock" with respect to isotope composition, as new radioactive isotopes readily diffuse in to replace the old ones. Therefore, their assumption seems to be well grounded.

As to the last question, it depends on what you mean by "know". Obviously we don't have planet-sized clouds of dust to play around with in laboratories, and we also don't really have enough observation time to see these things happen (even though I said they occur "quickly", not many processes on the scale of planets take less than a few thousand years to happen). So we will probably never see more than a snapshot of this evolution process directly. Then again, if you trust the simulations that people are doing nowadays (considering how much thought is generally put into those things, it's reasonable to do so), we seem to have a general idea of how long the collapse should take, which is on the order of a million years, starting from the phase where the dust has already collapsed into 1 km-wide bits: https://www.cfa.harvard.edu/~kenyon/pf/terra/t0/index.html.