Прочитайте предложения в видо-временной форме Past Perfect Continuous, задайте cпециальный вопрос, начиная со слов данных в скобках:
Образец:

It had been raining for an hour when they left. (How long …?)
How long had it been raining when they left?

1.They had been doing this work for several days when they got this offer.
(How long …?) 2. The clerks had been preparing the documents for an hour when their boss came. (How long …?) 3. We had been skiing for half an hour when the snowfall began. (How long …?) 4. I had been cooking for an hour when my friends came. (How long …?) 5. The children had been watching TV for two hours when mother switched it off.. (How long …?) 6.They had been discussing the contract since 10 a.m.when they came to agreement at last. (Since when …?) 7. He had been having a shower for ten minutes when this TV program began. (How long …?) 8. The car had been running well since August when the accident happened. (Since when …?) 9. By the time I finished my shopping I had been walking for three hours.(How long …?) 10. He had been playing tennis for some years before he could play well. (How long …?)