There are two schools of thought around when should you launch your white paper, which is either to launch early or launch late with your white paper.
Launch white paper early approach
Traditionally ICO projects would launch their white papers months in advance. The idea is to give the community time to scrutinise the paper and provide feedback. The white paper often will be changed over the course of time and has versioning. Often white papers are very technical and can take time perfect hence the more time that the community has to review the better.
In addition to the white paper the token code can be made open for review.
Launch white paper late approach
A new school of thought is to launch a few weeks before the ICO and to try to get attention and funding while token buyers have given the white paper a look over.
It is often said that only a small number of buyers in fact read the white paper. This makes sense as most buyers only buy small number of tokens and often will buy less than $200 of token value. With this in mind the incentive to spend to much time on reading a typical white paper that is over 20 pages makes sense.
Under this approach the ICO marketing will be very aggressive and be dominated by paid marketing and fear of missing out (FOMO) tactics.
Based on the marketing approach the timing of the release of the white paper can be figured out. The preico demand can also play a big part of the timing as if you have a high percentage of the ICO already filled out +50% then the white paper release becomes less important.