It really doesn't take that long. Pop them in and walk away and it seems like in an hour or so you walk by and the green light is on and they are good to go. It works great. It is also nice to have 2 in the camera and 2 in the charger.
You don't and they are probably NOT charged, as per my somewhat extensive testing. Don't deal with these people, as they discount returns you have to pay for. I have had good success with the more expensive but independent charging Sony. (You know when EACH battery is charged and they are charged). Synergy is SUPPOSED to charge in pairs, but supposedly doesn't account for unevenly used batteries. However it still didn't charge either of the pairs, so couldn't even prove or disprove this.
I would think it would work for the L310 as well but I am not sure. It does work great with my L810!! Make sure to go into the settings on the camera and select the "Coolpix" Battery setting for best optimization.