How long does it take for oral cancer to develop?

It can take long. Most patients present at over age 50. Tobacco use, including smokeless tobacco, is certainly a risk factor. There may be few or no symptoms. Mouth cancers can present as non healing ulcers that can be painful. Rarely, there can be painless lymph node enlargement. Patients are staged by size of tumor and lymph node involvement.
Usually many years. Oral cancer is generally an illness of people in their 50s and above, thus, it usually takes many years to develop.