Most wall chargers don't "shut off" per say. If you charged it overnight and it is showing 5.8v, its probably fine. But I would recommend getting a "Volt Watch" to check the battery condition after each flight. They are about $12.
I've got a couple planes that I equipped with a Volt Watch on board, so I can see right away how my battery is doing. I've also got one that I can just plug into the charge port or directly to the battery and check its condition. There are different types and brands available, but here's the one I use.

[link]http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHDJ2&P=ML[/link]