How long were the Germanic dialects commonly subsumed under the term "Frankish" spoken by Frankish people in Northern Gaul, and how long did it take until they were completely supplanted by the Romance dialects that would later evolve into French?
EDIT: I'm aware that Alsatian and Dutch are still spoken today, but only in parts of the country that were either not part of France until historically recently or had never had a Romance-speaking majority even before the fall of the Western Roman Empire or the defeat of Syagrius by Hlodwig. I am talking about the Frankish idioms in regions where Franks were a minority.
You have to distinguish three areas in Northern France (involving départements 59+62).
In 62, you have to distinguish between Boulonnais (west) and Artois (east).
In Artois, there never was a significant number of Germanic speakers. This is shown by toponymics and indirectly shown by the fact new evangelization was never necessary. So Artois is basically a stable Gallo-Roman (later French) area.
In Boulonnais, the situation is more complex as a significant number of Saxon (not Frankish) people settled in this area and created a huge number of hamlets and villages. It seems that this dialect of Saxon died in the 1200s, and was quite certainly dead by the 1300s. This area was a mixture (a patchwork) of Saxon and (dialectal) French speakers and you can tell who lives where, because the names of the villages do not evolve the same way (and do not have the same etymologies). Germanic names are quite conservative while French names undergo the regular attrition of French.
This area (Boulonnais) had to be reevangelized in the 6th or 7th century due to the high number of Germanic hence pagan people.
There were also pockets of Viking (like in Sangatte), Frankish (like in Frencq) and even Swabian people (like in Zouafque). But they were marginal. It seems that there were a few pockets of Germanic speakers living in farms further south in 80 as well. All these pockets probably switched to (Picard) French in the 1200s, which is the time when French began to expand and dissolve non-French areas.
In 59, the linguistic border between Flemish and non-Flemish was approximately stable for a very long time (since the Middle Ages) and French began to eliminate Flemish only in the last 150/200 years. Contrary to a claim made on wikipedia or some other places, 62 never spoke Flemish or Dutch at any time.