Basically, as detailed in the main Amex thread and the sticky; you ideally want your voucher to appear in your account the day before your redemption flights become available, (the 2-4-1 is only valid for 12 months and flights become available around 11 months ahead of departure), and if this is towards the end of the Amex year you need to be careful that you don't trigger your voucher early as it could expire before your departure date.

So I suspect many of us keep an accurate running total of our Amex accounts and even lock them away for 6 months in order to not trigger the voucher too soon!