For a patient with osteopenia or osteoporosis, how long can intravenous zoledronic acid be maintained if there are not adverse events?

I generally prescribe Zoledronate annually for three years for patients with osteoporosis who are at increased fracture risk. I will occasionally add an additional treatment 18-24 months after the third infusion if there has been a positive response to the original treatment regimen and I feel that ...