NRIs are not permitted to open new or invest in PO-MIS or any other PO accounts. Such prohibition applies to existing account for continued investments, as well as new accounts. You have to hold the funds in non-repatriable account in India. And inform PO about your new status (NRI).

Maturity period is 5 years, so after 5 years (as No tax rebate is applicable) it will be added to your income.