There wasn't any assigned cost to doing this, either in chi, PPE or time. I figure it would take at least 1 melee action, like most things...
I had at first assumed that they had to remain in the body to control the body... but it appears I was wrong about that.
If you look earlier on page 76 under the spell Turn Away the Dead...
if used against a group of dead animated by a Brass Snake, it will affect all of the corpses except the one actually occupied by the Brass Snake
This appears to indicate that once a Brass Snake enters a body and brings it to life under their control, they can choose to leave it and move on to reanimated another corpse... and another... and another... and the left-behind corpses will remain under that snake's control!
This is pretty terrifying because there are no stated limits to this ability. A single Brass Snake could bring to life an army of millions if
given enough time?
I was thinking as some sort of game balance, perhaps they should borrow the "Animated and Control Dead" rules for Undead Immortals from page 118. This would limit them to a maximum of 1 per level and require instilling 1 negative chi per each one. Otherwise, a single Brass Snake could unleash some kind of zombie apocalypse.
Then again... that might be fun.