"That is not possible to say in this situation," Kehm told German broadcaster RTL.
"Michael is making progress appropriate to the severity of his situation, but it will be a difficult and long process."
Schumacher suffered severe head injuries while ski-ing with his family last December in the French Alps, leaving him in a coma for six months.
But he has been recovering at his home in Gland, Switzerland since June.
Kehm again stressed that Schumacher's family are grateful for the constant flood of well wishes from fans of the seven-time world champion.
"I can only say again that the family is very happy and touched by the sympathetic messages," said Kehm.
"I believe that positive energy does them good. We appreciate the condolences from around the world."
Last month, a French doctor treating Schumacher echoed Kehm's message that the 45-year-old German Formula One ace was making progress, but will need years to fully recover.