Hi, sorry about your mare.
Have you spoke to your vet & farrier? What treatment have they suggested?
I don't think anyone will be able to give you an answer as to when your mare will or will not be able to be used.
Your mare, herself it the best person to tell you if enough is enough.
There's lots of different treatments available nowadays, corrective shoes, nerve blocks, various drugs or anti inflammatorys which act on the circulation to increase blood flow to the navicular bone.
Speak to your vet and farrier, they will be able to advise you of the best way forward.
Hope they can sort something out for you both.
Let us know how you go on. X