There's a huge sliding scale there, for a number of reasons. The skill difference between the ceiling and floor is not small, and Square seems to believe the skill floor that the game should support should stay quite low in most content. A highly skilled healer might spend a dungeon without casting a single GCD heal. An unskilled one might cast 50 or more. For just that one reason, we already have a clear variation between the amount of time spent healing between players. When the content and kits "need" to support taking dozens and dozens of casts in easier content, as a side-effect they end up supporting 80+% damage uptime for someone who doesn't need to do that.

I'd be okay with 40ish% of my casts being heals in hard content once I've mastered it, but I'd also like the healing kits to be interesting for that. I think the skill gap makes conversations about how much healing uptime is "enough" hard to reach a definitive answer. Making the healing and damage kits interactive and interesting to play on their own would make the wiggle room in that uptime percentage much more bearable, no matter where it lands. I wouldn't have fun spending 80% of my time healing if I'm just casting Cure Cure Cure Cure Cure Cure like I would Glare Glare Glare Glare Glare Glare Glare.