This might seem silly, but I think the best answer is that the majority of time should be devoted to those religions other than the locally dominate one. For example, in a Christian area it might be that the majority of time was spent on all the others and only a small portion on Christianity.
But religion should also be incorporated into almost every subject. Math for example is incomplete unless you understand the Islamic revolution in mathmaetics. In addition, any study of science would be incomplete without and understanding of the part that Islam played in maintaining science during the Dark Ages and then the rennaisance that swept up from the Caliphs of Spain into northern Europe.
Even a cusory glance at history will show that so much of it was driven by religious beliefs and so a study of religion must be part of every history class.
Aslan is not a Tame Lion