The timeline is based on the number of pages they propose to release (500 a month) and the fact that the group requested 329,000 pages of documents.
I would guess that since the law requires that the documents be released in a timely manner, some concessions will have to be made. They might have to hire a lot of people or assign a lot of people to that department for a month.
FOIA requests have to be manually reviewed to make sure that only data that can be released is released. Some info is free from FOIA, specifically personal data (patients names and addresses, for instance) and possibly any proprietary information that the government was provided but is not free to disclose due to NDA's.