Its very subjective thou, based on experience wall charger usually perform a faster charge comparing to USB cable charge.
No need unplug is also possible yo, Apple battery is rechargeable lithium-based, it will automatically stop charging when it reaches 100%. So chill, leave it there or unplugged, either way is fine.

Not quite correct.

I used to have the same problem. Couldn't figure when or why it happens and still don't. I do notice that it will happen everytime I upgraded the firmware.  Firstly to 4.3 and then to 4.3.1, 4.3.2 and 4.3.3.

What worked for me may not work for you.  The problem for me was that I keep the mobile data switched off (I prefer to use wifi).  So whenever the usage = standby time happens what I will do is switch on mobile data.  I will leave it on for a few minutes and then switch it off.  The standby time will now continue ticking but the usage time stops.  Works everytime so far.

My theory is that something seem to be running in the background trying to send/receive data using the mobile network.  Since I have it on off, it cannot do that and will go into a continuous loop which shows up as usage time even though the phone is in standby.  By switching on the mobile data, the problem code will be able to complete its data send/receive and shut down. So no more unintended usage.

I'm using an iphone 4 by the way.