asked the Postmaster-General what number of days is required for letters to reach Gibraltar by air-mail?

Correspondence on which air-mail fees have been paid should reach Gibraltar about 10 days after posting. There are, however, recurring difficulties about air capacity for mails on this route, and I regret to say that this time is frequently exceeded.

Is the average time about 10 days?

Yes, Sir. It has been as short as five days and as long as 20 days, but 10 days is the best average.