Learn English – How to use Present Perfect Continuous in Passive form

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.

It's never bad form to use passive form. It's just that in speech, we tend to use a lot of this, but there's nothing wrong with using the passive form in writing, or in speech.

From the Passive Engineer:

Despite the admonitions of grammar checkers, the passive construction has a legitimate function. When you want to emphasize results, use the passive.

Note that it mentions grammar checkers, which I suppose is what you are getting.

Wikipedia states that:

Many language critics and language-usage manuals discourage use of the passive voice....This advice is not usually found in older guides, emerging only in the first half of the twentieth century

Also:

In 1926, in the authoritative A Dictionary of Modern English Usage (1926), Henry W. Fowler recommended against transforming active voice forms into passive voice forms, because doing so "sometimes leads to bad grammar, false idiom, or clumsiness

It's really just style, but nothing else to worry about.