Learn English – “Rather than doing” vs. “rather than do”

You could say 'I'm waiting two hours', and no one would misunderstand you. I don't think there is any loss of meaning, but you might sound a little foreign. I think this usage is standard in some types of American-English. I could imagine a character on Seinfeld going 'Two hours I'm waiting - what is it with these people?’

In speech (but not in formal writing) you can say 'How long've you...'

Actually, there is a context where "I am waiting two hours" means something completely different (thanks Claudiu, Kosmonaut). If the issue was for example how long you were prepared to wait for a delayed flight, then you would say 'I'm waiting two hours (then I'm going home!)" to mean "I will wait two hours only"

So if the context is unclear, you need the 'have been waiting' construction.

The phrase "I can't hardly wait" is incorrect.

I suspect it is the result of a confusion between:

I can't wait

and

I can hardly wait

which are both correct.

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The phrase

I can't hardly wait

doesn't make sense: it would mean "I don't find it hard to wait", which is probably not what is meant.

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Probably adding to the confusion is the 1998 teen movie "Can't Hardly Wait". It is possible that the title itself was picked up because the expression is in vogue in American high schools although I did not find any confirmation for this hypothesis.