You or, more likely, your employer should keep making payments until the Portland Chapter 13 Trustee specifically requests that the payments cease. The Trustee will inform your employer directly when the payroll deductions are no longer required and should cease.
Depending on whether your income places you below or above Oregon’s median income level, you may be required to make payments to the Trustee for at least three years and perhaps as long as five years. It can be difficult to predict the exact outcome of your case when it is filed.
You should definitely ask your attorney whether you are below or above the median. That answer will likely tell you whether you must make payments for at least three years s or whether you must keep doing to for the full five years.
Finally, there is no need to be concerned about overpaying. Once your case has undergone final audit, the Portland Chapter 13 Trustee will refund any overpayment you might have made. It is far better to overpay a little than to underpay and risk having your case dismissed when it is near the end.