The FBI qualifies Freedom of Information Act requests for different processing times based on their size:
A request is routed through a small processing track if it encompasses 50 pages or less, a medium processing track if it encompasses 51 to 950 pages, or a large processing track if it encompasses 951 pages or more.
They have apparently quietly changed the rules to say that any request involving more than 50 pages of documents is now a large or complex request. This means all non-trivial requests will now have an expected turnaround time of 659 days.
Obviously it's a safe bet that any FBI investigation into Trump will have involved more than 50 pages of documents.