Excluding the Germanic areas of modern France, I would say there is only a small distinctiveness for these communities that has been part of another nation for some 300 years. Bretons, for instance, are not especially "non-French" because they have been French for centuries. One can be ancestrally Breton, but it is highly unlikely you can be ethnically Breton in any meaningful sense. Just as, for instance, Calais is not ethnically English anymore.
That would be like saying that the Irish are English because Ireland has belonged to England for centuries. Actually the parallel between Ireland and Brittany is startlingly similar now that I think of it. Both regions first came under Germanic influence with the Viking invasions in the 9th century (as well as the Franks in Brittany). The Norman English colonisation of Ireland started in the 12th century, which is the time when Brittany became a vassal duchy of France. However, Brittany only became a possession of the King of France in 1532, four years before Ireland was annexed to the English crown by Henry VIII. From the 18th century on, the English tried to suppress the Irish language to replace it by English, and the French did just the same with Breton at the same time. The two timelines match perfectly. This is why saying to a Breton that he is ethnically French sounds just as outrageous as to tell an Irish that he is English.
Thus why I said 300-500. Some areas are not especially good at becoming distinctly part of another ethnicity.
300 or 500 years doesn't make much difference. Most local identities and ethnicities in Europe go back at least 1000 years, but in some cases over 2000 years (e.g. Greek settlements in South Italy). We are back to ancient ethnicities like the Greeks, Romans, Celts and Germanics.
For Germany, eastern France and the Low Countries, I found that the stem duchies of the Holy Roman empire match pretty well the regional ethnic divisions (better than modern political boundaries anyway). For instance, the Duchy of Lower Lorraine encompassed most of modern Belgium, Luxembourg, the northern Rhineland and the southern Netherlands. It only misses the Counties of Hainaut and Flanders (ironically part of France back then) to make up the greater cultural region that evolved from the Frankish settlements in the Roman province of Gallia Belgica. So, this ethnico-cultural region has its roots in the late Antiquity and early Middle Ages.