One afternoon towards the end of summer Henry and his gang initiate a rock fight with the Losers in the Barrens. The bullies are left defeated and embarrassed - an injured Henry swears revenge on the Losers before departing. After further encounters with It in the form of Pennywise and various other manifestations, the Losers construct a makeshift American-Indian smokehole which Richie and Mike use to hallucinate It's origins. In doing so they discover that It came to Derry millions of years before in an asteroid-like impact, and that every 27 years It awakens from a slumber underneath the town's sewers, usually after some kind of terrible event or tragedy, to feed on children for a period of 12–16 months.
I cant remember the movie, but i swear they said 27 years too. Ill have to rewatch it before the new movie