The debt phase will last as long as is necessary to adequately pay back outstanding $SBOND 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 $SBOND 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 $SBOND. If the debt phase is ended too early, and then the treasury doesn't have enough $SCARAB to repay the $SBOND bonus, then the APR restriction would need to be turned back on.