Pretty much the same as what Captain Gh3y said...I write roughly 1200-1300 words in 40 minutes which is generally 4 or 5 A4 pages. It varies for BOS booklets depending on what my writing is like that day...
At the least I can do three and a half A4 pages, but usually I can do four if the question is a good one. It's harder to tell how many pages that is in a BOS booklet though