Hang on, Faith didn't say anything about Christ. I'm sure that everyone can be made happy by adding religion to schools. I'd guess that it should be apportioned by population figures with a minimum set so that everyone gets covered.
That'd mean about 25 % Catholic teaching, 10 % or so secular of one form or another, and so on. Where I live it would be around 35 to 40 % agnostisism/atheist/secular for starters.
Of course, I don't think that secularism is teaching my views either. THe atheist time would be taken actively attacking the idea of the existance of God.
I suppose that 5 to 10 instructional hours would be fair to the minority positions. I'm guessing that something like 1/3 to 1/2 of the various Christian sects would fall into this minimal group.
The US is about 75% Christian of one sort or another. It would be hard to figure out how to divide the instructional time up. Let's say there should be a single class unit of 3 hours per week for 3 years of high school. We have about 300 hours to work with.
There are going to be about 20 or 30 minor groups of under 1 percent. They'd take about 100 hours to cover.
The remaining 200 hours would have to be set to about 150 hours to Christians. Of the remaining 50 hours about 30 would to to secularists (but non atheist/agnostic if they are grouped separately which the stats I have do )
I'll edit the rest of this in later.