If France's population is 60 million today, then it won't be much more than 80 million in 30 years, since the French just aren't reproducing very much anymore. And most of that growth will come from immigration, which will likely continue to come mostly from Algeria. I think there might be as many as 40 million Muslims in France in 2036, which would be 50% of the population but still far less than 50% of the electorate since so many Muslims would be below 18. Even if the immigration rate slows down a bit in the short-term (obviously Muslim immigrants in the next couple of years will have a far greater effect on France's population than Muslim immigrants 25 years from now), I think France will have a Muslim majority in around 50 years.
But the first Western European country to have a Muslim majority will be the Netherlands, where already a majority of children are born to Muslim parents.
I thought that immigration into France had slowed a great deal. Certainly, if the trends point to a Muslim majority, the French aren't going to encourage further immigration along the lines of the 1960s or even today, and sentiment for expulsions and exclusion will harden. We've been seeing a backlash for at least a decade, and it's not as easy to get into France as it is to cross from Mexico to the U.S. And certainly there are many factors driving Muslim fertility downward in France.
It is exceptionally tricky to project current trends outward to show a minority becoming a majority. There's too much time for the people in power to react and change things, never mind for the minority to change.
Do you have a link to that statistic about the Netherlands? I did a Google search and found one site called "jihad watch" which had some of "dhimmi" index, which I don't consider too reliable. The Netherlands itself reports that there are 1 million Muslims in that country, 5.8% of the population.
I would be shocked if 5.8% of the population were capable of producing 50+% of births, no matter how much the Dutch seem to have given up on procreating. Certainly if the Muslim community is having that many children, a good share of that 6% wouldn't even be old enough to have children themselves!
I remember hearing that a majority of kids born in Rotterdam were Muslim; perhaps that's what you meant.
No matter if it's 25 or 50 years, I think Europe is in for a mess. Perhaps the Muslims will "peacefully" conquer Europe this time, unlike their atrocious military campaigns in the 8th century.