This ACS article claims that most people will see cognitive function return to normal after 1-2 years. I wonder if these "most people" are the ones who were not treated with the high-dose protocols for bone cancer?
Here's an interesting article about breast cancer patients having cognitive issues before they even have the chemotherapy. It might make sense that the stress of the event is causing some of these issues. Also, depression and anxiety are known to affect cognitive abilities.
This study seems to show that breast cancer patients who received the high-dose chemo drugs (like most bone cancer patients have) had more cognitive problems after two years, but still those problems appeared to be transient and improved with time.
This article about late effects of childhood treatment seems to claim that high-dose methotrexate can be a culprit in cognitive issues after treatment.
A final article I read stated that there just aren't enough studies out there to show which chemotherapies are causing cognitive deficits, how bad they really are, etc.