I have one of those in my Ranger that I leave plugged in. After a three week Texas vacation, it started right up when we returned. The drain is minimal as pg318 stated; far less than the security system which is on all of the time.

 

I have gotten away with leaving my inverter turned on all weekend; but, sometimes leaving the headlights on for (using the switch) for 20 minutes will kill the battery.

The USB adapter reduces the 12V to 5V, so less drain than you might assume.

If there is no load on the adapter there is no drain.

 

except for the power light, if he has one

 

Even with a power light, which mine has, there's negligible drain. It's an LED.

 

I do the same.

AND
I left phones charging overnite OR
a (small) GPS on overnite no problem

 

My adapter has 2 USB outlets but thankfully no LED. The only thing I don't like about it is that sometimes it comes out of the socket by a few millimeters and loses connections. I constantly have to make sure it's plugged in all the way.