There is no optimal time. The plan involves a German fleet large enough to defeat the USN (and with a sufficiently large embarked army) transiting waters dominated by the RN during a period when the British public was very paranoid about the possibility of invasion. To be precise, there are two possible routes: first, the North Sea, which will only worry Britain and Scandinavia; second, the Channel, which will have the French in as much of a fever-pitch of excitement as the British. Even if they don't act, the Americans will have forewarning of the coming attack.
Whether this enables them to defeat the coming German attack is something covered in a few previous threads on this topic, with views depending both on facts, personal opinion and the precise date. But the long-term impact on German prospects is bad, since even a successful campaign will see American industrial might turned toward military and naval ends, if only to prevent a repetition of any defeat,
and her alignment with the Franco-Russian Entente. So in the event of a war akin to WWI, American national sentiment will be inclined against Germany from the beginning, rather than being pretty ambivalent as IOTL.
If you check out the search function, you should find a couple of threads on this discussing possible scenarios in greater detail than I've outline above.
