Historically, how quickly have ice ages occurred or disappeared?

The peak of the last glaciation (the Pleistocene glaciation) was roughly 20,000 years ago, with about 97% of Canada totally covered in thousands of feet of ice.

An ice age can occur if the average daily temperature drops by only a few degrees Celsius for an extensive period. Ice ages include colder and warmer fluctuations. During colder intervals (glacial periods), glaciers and ice sheets grow and advance. As the snow gets deeper and deeper, the lower portion turns to ice and its incredible weight makes the ice sheet flow across the land like a slow, apparently solid river. In warmer intervals (interglacial periods), glaciers and ice sheets shrink and retreat.

We're in an ice age *right now.* It started about 2 million years ago and is known as the Quaternary Period. Despite the many warm periods since then, we geologists still consider ourselves to be in an ice age because of the continuous existence of at least one large ice sheet - the one over Antarctica. The glaciers and the Greenland ice sheet are also of long standing, but they are more recent. We are currently enjoying a warm interval: our climate represents an interglacial period that began about 10,000 years ago. The preceding glacial period lasted about 80,000 years.

At least seven ice ages have been recognized over geologic time. At least four of them are considered significant because of the extent of their glaciation or because they lasted for an extremely long time:

* about 2 million years ago to the present—the Quaternary Ice Age

* 350 to 250 million years ago—the Karoo Ice Age

* 800 to 600 million years ago—the Cryogenian (or Sturtian-Varangian) Ice Age

* 2400 to 2100 million years ago—the Huronian Ice Age.

The timing of ice ages is primarily controlled by what is known as the Milankovitch cycles, an orderly, predictable perturbation in Earth's orbit that sometimes takes us either unusually close, or unusually far from the Sun.

No history - the end of the last ice age predates all human memory or record.

Currentlly it is thought that ice aes start and finish pretty quickly, the temperature flip taking about 200 to 300 years. They last over 10'000 years (well over usually) and the interglacial periods average around 10'000 years - and it's 10'000 since this interglacial period started.

Earthman has it all right, but I feel like I should say one more thing. Typically ice ages begin very slowly, and end very quickly (on a geological scale that is). It is due to the positive/negative feedback loops partially due to the albedo effect of snow on sunlight/heat.

I'm pretty sure Ice Ages last for a couple thousand years.