Learn English – How to say: after have waited or after waiting e.g. 4 hours

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.

In my (UK) idiolect "have a rest" and "get some rest" are idiomatic, not the two phrases you give.

I find a slight difference in meaning. To me, "have a rest" is neutral: I am going to rest, with no implication about how much I need it.

"Get some rest" implies that I have a particular need for rest: either I have been short of rest recently, or some major activity is expected so that I need to be well-rested before it, or there is very little time available for resting, so I will avail myself of it.