after the fall patch it seems to have increased dramatically (passive religion). I was playing a game as Russia testing religion on Deity and my 6 cities got my religion without me using missionaries / apostles. Also my neighbor, Teddy, also was passively converted to my religion (this was without any religious combat or anything happening near the border, opposite of Teddy (who was situated between me and all the other civs in the game) was a separate religion founded by Arabia).
Edit- it's also worth noting that your religious units will heal while in YOUR holy sites, but won't heal in other civ's holy sites even if they are converted to your religion. Seems to heal 50% per turn.