There is no general prohibition on using "for + time period" (or, indeed, other time-period adverbials) with the past progressive. We were chatting for hours. She was living there for a long time.
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hanuman_20001. She was writing that play for three years.Here timed period has been used with the past progressive, which I could not understand.Yes, that sentence does seem anomalous — to my ear, anyway. I'd say "She worked on that play for three years".
hanuman_2000Mostly time adverbials denoting the time period are not used in the
CalifJimYes, that sentence does seem anomalous — to my ear, anyway.Hmm, I am surprised by that.
GPYHmm, I am surprised by that.https://books.google.com/ngrams/graph?content=walked+for+an+hour%2Cwas+walking+for+an+h