How often do health globes spawn?

Solution 1:

Blizzard has recently published some data on this on the official guide:

• Normal monsters: chance to drop a health globe on death varies by monster type.
• Champion monsters: 60% chance to drop a health globe at 50% life and on death.
• Rare monsters: 100% chance to drop a health globe at 50% life and on death.

In addition, with the superunique and act bosses, you can look at their health bar to see when a globe might drop:

The triangles in monsters’ health bars represent points where they might drop a globe

Regarding the "varying chance" normal monsters have to drop, typically the stronger the individual monster is, the higher the chance. Monsters that come in groups and are easily felled have a relatively low chance of dropping, while solitary, tough monsters - for example living trees, or those huge wall-climbing siege monsters in Act 3, are guaranteed to drop a globe upon death.

Solution 2:

According to the Brady guide, normal monsters have a "varying chance" to drop small health globes that will restore 20% of your maximum health. Champions have "a chance" to drop two globes worth 35% health each, and Rare monsters will always drop four globes worth 35% health each.

More specifically, the actual drop chance varies with each specific monster breed, generally corresponding to the monster's toughness. For example, Fallen Grunts have a 5% drop rate, Fallen Hellhounds have a 12% drop rate, Fallen Maniacs have a 20% drop rate, and Fallen Masters have an 80% drop rate.