Fashion is an ongoing conversation. This is not your typical 9 to 5 corporate job. As I said, the difference between the fashion industry and others industry is that you can never really have a separation.
1 month is enough to prepare the runway show. In those big companies, a lot can be done in a week. Even more than concepts and ideas may have already been thought about as many collections and projects have been in work since the last show.
You can think about the next show right after showing but things really materialize in the last month.