Hope the following answers your question.
The House of Lords Science and Technology Committee held an inquiry into air travel and health in 2000. Their findings on the issue of recirculation of air and the transmission of disease are set out below:
1.61 The typical cabin air flow of 20 cfm of air per occupant equates to a full change of cabin air every 2 to 3 minutes, i.e. 20 to 30 times per hour. As half of the air being changed is re-circulated cabin air, this is equivalent to an entire exchange of cabin air with fresh air 10 to 15 times per hour. (Paragraph 5.5)