If you have no refined materials production pauses, with the ultimate result being that the original 12 days becomes something longer than that. If you encounter this problem a lot you should build more refineries.
Build more refineries IF
you have a surplus of RAW MATERIALS; otherwise you need to build more mines AND refineries
Refineries only work on RAW MATERIALS, so if you don't have any RAW MATERIAL surplus then extra refineries just waste your time and resources. Optimally you want to have an equal number of mines and refineries, for maintenance points and to minimize RAW MATERIAL surplus. Having a REFINED MATERIALS surplus is a HUGE bonus (anything you want to build, within limits, would take the minimum amount of time to build).
So: 1) try to have an equal number of mines and refineries, 2) if you have a RAW MATERIAL surplus build more refineries (when the surplus is gone goto rule 1), 3) if production drags build MORE mines and refineries.