They take into consideration all the new toons and their need to run tasks, so it's in intervals.
It's varied from year to year, but I've seen the first invasion start around 10 PDT / 1pm EDT *OR* 9PDT / noon EDT. It would run for about three hours then stop or a few before starting up again.
With smusher my friend and I planned out the tasks of TTC and tried to get through the cog-specific ones as quickly as possible, leaving gag track ones for during the invasion.
As promised a couple of weeks ago, I will help new toons who are in "our district" so be sure to head there with your Stratics friends after you've done your trolley thing.
The invasion is only for Halloween. The day begins late with the site down for maintenance to make the changes, and pretty much ends that way as well. On November 1 your punkin head will be gone with a BAMF noise while the scene loads.