I'll only speak for myself. I teach in a public school, I teach comparative religions within ancient history. One must go over polytheism, Christianity, Judaism, Islam, Hinduism, Buddhism, and Confucionism. I happen to teach them when dealing with, in order: Greece/Rome, Middle Ages, next two are Mesopotamia, India for two, and China. In each case, religion takes 1/2 to 1 lesson-45 minutes. It's only the basics: A large religious difference also appeared in the area of Mesopotamia, monotheism...Teach Judaism-Moses and Abraham-covenant and commandments-Torah. That's it. For Islam: 5 pillars and the founder was Mohammad, no god but 'Allah'-Koran. One of the three monotheistic. Buddhism: Seach for ending of pain; not so much a religion as a philosophy-just try to have 11 year olds get that distinction-karma & motorcycles, no not really! Hinduism: striving for the right life-Dharma. Confucisionism: do the right thing; live a good life; do not bring shame-Confucious and some of his sayings.
Catholocism is dealt with in religion-where it belongs. In social studies, Christianity is taught with the same basic information as the others, of course the students are more familiar with it, so it takes much less time.
BTW, atheism and agnosticism are dealt with in religion, not social studies.