Looking at the last 2 years of flying NW as a KL Flying Blue Plat, with the exception of one SFO-MSP red-eye, the only NW flights I have not been upgraded on were ones where the ticket was bought less than 1-2 weeks prior to travelling. Almost invariably tickets bought 1-2 months in advance were upgraded like clockwork at the Plat window, with the occasional FC BP being handed to me before boarding.

I understand that NW treats FB Plats equally in domestic EUA process. If years as Plat counted, that would have trumped the ticket purchase time given that I have been Plat for more than 15 years.