Stress Fracture in Tibia (just under knee)

I’ve just been diagnosed (after being in pain for 3 months) with having a stress facture right at the top of my tibia, just underneath my knee joint. I’ve already been on crutches for a month and the pain hasn’t got any better, although apparently, the fracture is healing OK. My doctor wouldn’t give me any idea how long it would take to heal. Has anyone had the same injury (apparently it’s pretty rare) and can you give me any idea how long I can expect to be on crutches for?