How long can a whale stay under water and why?

I thought this was interesting ~~~>
What is the difference between dolphins and whales?

There is a difference between what we call a whale and what biologically is a whale. We tend to use whale for larger mammals living in the sea. Whales have baleen whereas dolphins have teeth. Killer whales for instance therefore are technically dolphins.

How they are able stay under water:

Whales breathe through blowholes, located on the top of the head so the animal can remain submerged. Baleen whales have two; toothed whales have one. The shapes of whales' spouts when exhaling after a dive, when seen from the right angle, differ between species. Whales have a unique respiratory system that lets them stay underwater for long periods of time without taking in oxygen. Some whales, such as the Sperm Whale, can stay underwater for up to two hours holding a single breath. The Blue Whale is the largest known mammal that has ever lived, and the largest living animal, at up to 30 m (93ft) long and 180 tons

Because of their environment (and unlike many animals), whales are conscious breathers: they decide when to breathe.

How long can a whale stay underwater?

They mostly stay underwater for 3 to 8 minutes. However, longer periods of up to 30 minutes have been recorded

The Bottlenose whales and Sperm whales can stay underwater for almost 2 hours on long dives. Most of the large whales, like the Blue whale and the Fin whales, rarely stay down longer than 40 minutes while large dolphins such as the Bottlenose dolphin usually stay down for less than 15 minutes. The smaller Common dolphin dives for less than 3 minutes.

The longest recorded dive by a whale (Guinness Book of Records) was that of a Sperm Whale. On 25th August 1969 a bull Sperm Whale surfaced from a dive lasting 1 hour 52 minutes
15 to 25 minutes to look for food
depends on what whale
I believe the answer to this question is 8 minutes and it has something to do with the temperature of the water oh wait thats dolphins Sorry
It depends on the whale. I think the record-holder is probably the sperm whale, and the record is at least an hour. They've just evolved to be very good at holding their breaths, because they need to in order to survive well in the ocean.
Forever if its dead
Big Lungs
the botlenose whales and sperm whales can stay underwater for almost 2 hours on long dives.

because they take oxygen from the air. as because ther are in the mammalian family.
Lots of very good answers but they mostly seem to overlook the other meaning of why. Probably not to seem patronising though so respect for that.

As for the question "why?", I would propose this response; Whales are mammals and therefore need to breath air as humans do.

Therefore as humans can hold their breaths for differing periods so do whales, both between species and between individuals within a species there are differences.

If you had to spend your life in water, with all your food below the surface, you too would develop the ability to hold your breath for long periods, natural selection would favour any children you produced with greater lung capacity as they would be more adept at finding food. So, in a few million years all your descendants would have abilities similar to that of the whale.

As far as time goes, the others have given some good answers, all I could do would be to re-iterate those answers, which I won't.

as long as it likes
Sperm whales are the deepest divers and can dive for over an hour and a half.
Deep-diving whales actually have small lungs. But they use air much more efficiently than, for example, humans. Their blood has a 50% higher level of hemoglobin and so have a higher capacity to transport oxygen. We use just 10-20% of breathed air - whales use 80-90%.
When we dive the oxygen comes 34% from lungs, 41% from blood and 25% from muscles. For whales it is 9, 41, 50 %.

In all land animals it is just the windpipe that have supporting rings. Whales have this supporting structure on the smallest branches of their bronchial tree.

During a dive the whale's heart beats at half the speed that it does on the surface, and the bloodstream to non-essential parts of the body is turned off by special muscles, cutting off oxygen use where not needed during the dive.

It is nothing to do with evolution! Whales have been ingeniously and wonderfully designed to do what they do.
4 as long as it wants, it has the abilities of a fish.