The debt phase will last as long as is necessary to adequately pay back outstanding $DBOND debt. Please keep in mind that the DAO will also need to collect a little extra, as there needs to be a cushion to cover the bonuses when people redeem $DBOND over peg.
There's no exact way of calculating how many epochs it takes, since we don't know exactly when people will redeem their $DBOND. If the debt phase is ended too early, and then the treasury doesn't have enough $DIBS to repay the $DBOND bonus, then the APR restriction would need to be turned back on.