There’s no certain number or goal that’s asked. It’s simply people complaining and kicking in groups if healers aren’t doing “enough DPS” for the group. Does it happen all the time? Of course not. But I know many people.who have seen it happen from lowbie dungeons all the way up to LFR.
It’s been a weird obsession of some toxic groups for a while now. That’s the weird expectation that’s being discussed. Not some guild group or something giving healers a specific number to reach.