By your T-scores (obtained through a Dexa scan) and previous fracture history, primarily. Family history would be another.

From the article I linked:
Low risk of fracture: treatment is not needed. If a bisphosphonate has been prescribed, it should be discontinued and not restarted unless/until the patient meets treatment guidelines. Example: 54-year-old woman, menopause at age 51, lowest T-score –1.5, no risk factors, bisphosphonate therapy for 3 years. Treatment was not indicated in the first place and can be discontinued.

Mild risk of fracture: treat with bisphosphonate for 3–5 years, then stop. The ‘drug holiday’ can be continued until there is significant loss of BMD (i.e. more than the least significant change as determined by the testing center) or the patient has a fracture, whichever comes first. Example: 68-year-old woman, menopause at age 50, initial lowest T-score –2.3, parent with a hip fracture, bisphosphonate treatment for 5 years, BMD stable over that time. Treatment was indicated, but a drug holiday might be considered after 5 years of treatment.

Moderate risk of fracture: treat with bisphosphonate for 5–10 years, offer a ‘drug holiday’ of 3–5 years or until there is significant loss of BMD or the patient has a fracture, whichever comes first. Example: 72-year-old woman, menopause at age 48, lowest initial T-score –2.8, no risk factors, bisphosphonate therapy for 7 years, BMD increased over that time so lowest T-score now is –2.3. Treatment was indicated but after 7 years of treatment, a drug holiday might be considered.

High risk of fracture (fractures, corticosteroid therapy, very low BMD): treat with bisphosphonate for 10 years, offer a ‘drug holiday’ of 1–2 years, until there is significant loss of BMD or the patient has a fracture, whichever comes first. A nonbisphosphonate treatment (e.g. raloxifene or teriparatide) may be offered during the ‘holiday’ from the bisphosphonate. Example: 75-year-old woman, menopause at age 45, lowest initial T-score –3.6, rheumatoid arthritis requiring ongoing corticosteroid therapy for 12 years, two vertebral fractures by vertebral fracture assessment (VFA), treatment with bisphosphonate therapy for 10 years. Treatment was indicated and she remains at high risk of fracture after 10 years. If a holiday from the bisphosphonate is considered, interval treatment with teriparatide or raloxifene would be prudent.