Well my point of view as always been...1 hour minimum per day. If you can do more, great ! But I think that you really need 1 hour min per day to sink in enought information to move foward at a reasonable speed.
But then again, it all comes down to " whats your goal in music ? ".
If you only want to learn song and play them around a camp fire or with friends, you dont need as much practice time then someone who wants to understand and write music.