I find that because most if not all ingame gms are usa based then the best time would be like uk/europa time minus -5 to 8 hours ....
So i would say early afternoon to early evening uk/europe time
I have had sevaral gm responses within 20 minutes in this time frame...very helpfull too on all occasions....its a mith that u cant get a gm to help u juist need to hit the usa playtime mate