That depends. Site-finished floors will take longer to install than factory-finished floors since the finish needs to be applied, and dry, on site.
Depending on the type of finish used, you can expect that there will be multiple coats applied, and that each coat will need to be sanded before the next coat is applied, and also will need to dry thoroughly before the floor can be walked on. In addition, all wood flooring, whether job-site finished or factory-finished, will need to be delivered to the job site and allowed to acclimate for a period of time before the installation can begin. This can take several days depending on the material being used.
This is a very important part of the installation process because the wood must reach equilibrium moisture content (EMC) with the job site conditions to ensure a long-lasting, high-quality installation.