Currently, the Access to Information Act provides that the institution has 30 days to request an extension. The problem is that, quite often, 30 days is not enough time for access to information units to get enough information to determine how many additional hours will be required, given, as you say, the number of pages.
Often, units within a department do not even respond to the access to information unit within 30 days. It is therefore very difficult for them to make appropriate and reasonable requests for extensions, and that is often what leads to complaints to the Office of the Information Commissioner. The number of pages and documents is certainly an issue, but it's hard to know whether the Access to Information Act or the mismanagement of those documents is at fault.