Sorry for having missed the original question. I must have been away travelling (oh, no wait...)
There is no strict 10-year rule, but it was found that while the original CDs covering magazines and books from the early years of Air-Britain sold well, the demand for the later issues was much less and it became less commercially viable to produce them. We are looking at other ways in which certain elements can be shared.
As stated in my previous response, there has to be a commercially viable case to produce the CDs. When we carried out the last full stock take at the warehouse, we did not count the stock of CDs due to the large volume still sitting on the shelves. To produce smaller quantities means an increase in price, which leads to being less attractive to purchasers.