I imagine a lot of heal supports are gonna see heavy decreases in their healing stats

Probably 2k-7k for most games that go 20-40 minutes. Super healers like Taric, Sona, Soraka will go down by 8k-15k.

Of course a lot of this depends on team comps and whether or not your team could utilize ardent a lot. The lowest numbers should be the cases where just the marksmen could use ardent. Lower numbers on super healers are for pessimal team compositions but still could give ardent to their whole team consistently.

This is just from my experience with the health drain version of ardent censer and the amount of healing I saw in my games from that item alone.