How soon can bone cancer be diagnosed, if you have no bone pains?

Not sure. Hello alcides7. Can you elaborate? What symptoms or signs do you have that make you think you have bone cancer? Primary cancers of bone are unusual, but cancers can metastasize from other sites to bones (such as prostate cancer). There are also bone marrow cancers.
Very soon bone scan. Nuclear bone scans can diagnose bone cancer long before patient feels pain.