The line is usually crossed when monetization starts affecting what users came for in the first place. If ads interrupt conversations too aggressively, or key features feel locked behind payment, people start to disengage. But when monetization stays clearly optional and doesn’t interfere with core participation, trust tends to hold.
Some communities do manage it well. The ones that work usually keep the main experience free and clean, then monetize through things like voluntary subscriptions, or cosmetic upgrades.