I doubt that the Muslims would have conquered Europe because they were already very overextended. After conquering France, where do they go next? If they go to Germany, they are going very deep into Europe and overextending themselves even more. As far as I see it, the deeper into Germany they go, the more they expose themselves to enemies, I could see them making a lot of progress but I don't think it'd last long. Italy is better protected with the Alps and the terrain is more mountainous. That said, I could see permanent Muslim strongholds surviving in Italy for centuries, and not just Sicily. Britain at the time wouldn't really be worth invading, although it could be a threat to Muslim France to have an untouched Christian stronghold across the sea. Plus as the Vikings rise, Muslim Europe would face even more pressure.
Long term, the main problem for Muslim France is the defensibility of its borders. The Alps are good, but the whole north-east is exposed to the North European plains. All it would take is a strong power to rise in Germany and for Muslim armies to be occupied elsewhere and northern France would likely fall.
How would a strong power rise in Germany, though? Bear in mind that the catalyst for the creation of the Holy Roman Empire (aka, the First Reich) were the conquests by the greatest Frankish king in history, Charlemagne. Absent that, who would take his place with the Franks subsumed in an expanded Al-Andalus? Even in OTL, it took centuries for the squabbling Christian kingdoms in northern Spain that fought each other at least as much as they fought the Muslims to eventually retake the peninsula. And there were rarely more than a handful of them.
Imagine how much longer it would take for the even greater number of feuding Germanic tribes to reduce the Muslim presence in France to the Mediterranean region, if they even made it that far.
No, I think we would see a Muslim France and Spain (not to mention Portugal) surviving to this very day.
I don't really know when or how nessessarily, and that doesn't really matter because whether the Germans got their act together and attacked fairly soon after conquest, or waited severall hundred years, most of Muslim France's vulnerabilities would still exist and in my view, it would be much more vulnerable than Muslim Spain was OTL.
1. Muslim France would be surrounded by hostile Christian powers who would see it as a threat and want to defeat it. There'd be significant motivation for alliances to be formed between various tribes and states along the border.
2. France, especially Northern France would be isolated from the rest of the Muslim world. While various Berber Empires would cross the straight of Gibraltar and enter central and southern Spain very easily, France would be a much further distance for North Africans, let alone any support from the Middle East.
3. Northern France is mostly flat plains, wide open for any invading force. Spain, meanwhile, is much more mountainous.
4. While the reconquista was mounted from the mountainous regions of Northern Spain, this reconquest could be mounted from the Rhineland, which is clearly a more powerful starting point, even if the Germanic tribes were too divided.
And its not just the Germans that the Muslim French would have to defend against, the Vikings would also likely be invading the region and even the English may cause trouble.
