The pre-production phase can be long or short, this depends entirely on budget and the nature of the project. But, however long, it is the phase leading to production. It is when research is conducted, interviewees and locations found, staff hired, and schedules planned in detail.
At the end of this phase, and just before production starts, your producer should know who will be interviewed and where and when. Research should also have given them a good sense of what the interviewees are likely to say in relation to the original brief.
At this point is is worth asking the producer to submit a shooting script. Not only will this reflect the reality of what they expect to shoot but for you, the commissioner, it will (or should) give you a sense of story structure and how well it reflects your original brief.
Tempting as it is, I would, however, caution against seeing the shooting script as a document to be set in stone. It is a map, it will get you to the final destination, but there may well be unforeseen detours en route.