Personally, I try not to let the tank get below 50%. But it depends on a lot on how much damage the tank is taking and how fast their health is going down.
As soon as the tank gets enmity, I open with Divine Seal (if up) and Regen. If there are several groups of mobs I might pop in Eye for an Eye. This is usually enough that I don't need to heal for the rest of the fight.

If the tank's health is going down fast, I'll DPS until it gets to 30% or so. If up, Swiftcast and Cure II. Otherwise there is no time to DPS...
Then switch out of CS, heal him up real fast, and switch back to CS.

If his health is going down really slowly, there may not even be a need to heal at all. When it gets to 50 I'll pop in 1 Cure II and that's enough to bring it almost to full, without overhealing. Switch back to CS and DPS. (Because there is no CS cooldown on switching into CS, only out of it)

If the tank isn't using any cooldowns, I'll wait until their health gets really low to force them to actually play their role. I'm not going to be the only one working like crazy. We both need to play our roles effectively to clear the dungeon quickly and smoothly. That means proper use of skills.

But by and large, most tanks I'm with get to 50% slowly enough that I can do a lot of DPS, switch, heal, switch back, DPS more.
I don't want to the tank thinking "Come on healer! Hurry up! I'm about to die! Don't you see my health? Where are the heals?" At any point in the dungeon.