"during which period" is correct and usable in written English.
You can use it when you need to specify a particular period of time. For example, "The team won three consecutive championships during which period they remained undefeated."
In Italy, that could easily take 12 months, during which period the other count would have been timed out.
Today's share price hike adds to the 33% jump over the last year, during which period the wider FTSE100 has risen 3%.
"The next few years will be a very exciting time for McLaren, during which period we intend to become an ever-stronger technological and economic force".
The siege of Antioch lasted more than seven months, during which period many men deserted and many others died of hunger.
Herod reigned for over 30 years, during which period Jerusalem reached its peak of greatness, growing in wealth and expanding even beyond the new double line of walls.
From 1892 to 1896 Thompson lived near a Franciscan priory in north Wales, during which period he wrote Sister Songs (1895) and New Poems (1897).
The fort was captured by the French in 1686 and remained under their control until 1713, during which period it was called Fort-Saint-Jacques.
He was then its joint managing director from 1939 to 1943, during which period he was also a director of the Press Association and of Reuters , Ltd
The scholarship is tenable for one year, during which period it will be the duty of the student to devote himself to research in Cambridge or elsewhere.
Wagner kept the piano for ten years, during which period he finished Siegfried, wrote most of the final score of Die Götterdämmerung, and completed Die Meistersinger.
