What is the rehab like following mandibular / maxillary injuries?

Depends... This depends upon the severity of the injury; was an operative procerdure involved?, etc. Best to pose this question to the physician that care for you and/or the physical therapist involved in your care.
It depends. If you had a fracture, the nature of the rehabilitation depends on the type of surgery used to repair. If you had hardware placed, you will usually go back to chewing soon. If treated with just a wired jaw, usually wait 6 weeks before beginning to chew.