Quote from diutinusfides:
Varies. Assuming you are day trading. Always good to wait and see the London open around 2am ET and see if they don't play around with it a bit. First 45 mins of NYMEX pit can be tough also.
If you are more concerned about getting size filled, well than just watch the volume chart and see when there is size. Typically can do abotu 10% of total daily volume without moving price too much and losing slippage.