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Are all these sentences correct?

In the afternoon he swims for about two hours.
In the afternoon he swims about two hours.
In the afternoon he swims for two hours.
In the afternoon he swims two hours.

Thanks a lot.
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They're all good.

It's a little unusual in my experience to hear the last one by itself. I'd expect, "In the afternoon he swims two hours and then goes home to prepare dinner." For the single statement, I'd expect one of the other three. But I don't see anything wrong with it.
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My "problem" is precisely the last sentence.
Is it correct to express duration / length of time without using for / about / since...

It sounds a bit strange to me, but I would like to know if I can consider it gramatically correct. If so, can anyone give me other examples of the same structure?
Expressions like, "I boiled the eggs three minutes" are common, but tend to be casual or informal. (lower register)
Thank you!
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