grammer help "どれ ぐらい"

[lazysunday]

毎日　どれ　ぐらい　ねます　か。

can someone explain the grammar in this sentence?

Why どれ?

I thought どれ meant "which one" but

Everyday approximately which one do you sleep?

Doesn't make any sense. I have looked up どれ and cannot find an instance where it means "how long" but I am assuming it does.

Can anyone clarify?

http://www.spurrymoses.com/jlpt/4/ <--item 34

[Psy]

どれ on its own does mean "which" but in the construction どれぐらい as a whole means "how long" or "how far." So as a whole the sentence means "how much do you sleep every night?"

As for how this usage evolved, I have no idea.

[hatch_jp]

In addition to "how long" and "how far.", "どのぐらい" means "How much", "How many", "How big", "How fast" and "How often"

[WalterWills]

I think it's used for quantity and units...but not for things that can be physically counted, in which case いくつ is used.