Can deadlines for PhD admission in US be approximated?
It varies greatly. 


There are the fixed deadline places, for which there are deadlines starting from Nov 15 or thereabouts, all the way upto Jan 15.
There are also places with rolling deadlines, where it's better to apply early. 


In general, you should have your materials ready to go in the beginning/middle of November and that should take care of most deadlines.