It will not even take that long.
The Christian church in the West is shrinking at a rate that is truly astounding, and a large number of Christians who claim to be of the faith are lukewarm at best.
If Christians would not stand up to protect prayer in school, or protect Christian displays of faith during holidays, or stand against abortion and homosexual marriage - what makes anyone believe they are going to stand up for their faith when a knife is at their throats?
Chrislam is one of the major growing movements within the church in the USA right now. It won't take until the end of the century for Islam to become dominant. Much, much sooner.