I started leaving my console on for this achievement after I had got the other 960G.
I kept track of over 100 hours of leaving the console on and this still didn't unlock. 100 hours is not including all the time I actually spent playing the game and getting all of the other achievements.
I then realised that I hadn't re-installed the online update after finishing doing the survival wins achievements. (Removing the update makes the 100 wins a little easier) ;-)
I re-installed the update, entered a sparring match, then quit straight away and the achievement unlocked! It seems that not having the online update installed was blocking the achievement from unlocking.
I would recommend making sure the update is installed when going for this achievement so that you don't waste too much of your time.
The update that I mean is the online update. If you have a classic version of this game then the update is included and cannot be removed.
Another thing that I noticed is that if I left the console overnight in sparring, sometimes when I returned to the main menu in the morning "Now Saving" was NOT displayed, so the time played may not have counted. I switched to watch mode and everytime "Now saving" WAS displayed when I quit to the main menu after leaving it on.
If you feel any improvements could be made to my guide then please comment with your suggestions.