How long after a positive lymph node biopsy is it safe to assume cancer is everywhere?

No answer. A tumor with spread to local lymph node may be curable and cancer need not have spread everywhere. The risk of cancer spreading in the body increases with the incresiing number of lymph nodes being positive. However, there is no time line for the spread of cancer. Different tumors and people behave differently.
Alternate View. Many cancers have the potential to spread throughout the body via the lymphatic circulation. When doing cancer operations, we often remove some of the adjacent lymph nodes to give us a "window into the rest of the body"--if cancer cells are present in the ln, we can assume that cancer cells may be elsewhere. This doesn't make the cancer incurable; it just means that chemorx may be necessary.