The time required to roll back a parallel DML operation is roughly equal to the time it takes to perform the forward operation.
Oracle Database supports parallel rollback after transaction and process failures, and after instance and system failures. Oracle Database can parallelize both the rolling forward stage and the rolling back stage of transaction recovery.
Oracle Database Backup and Recovery User's Guide for details about parallel rollback
A user-issued rollback in a transaction failure due to statement error is performed in parallel by the parallel execution coordinator and the parallel execution servers. The rollback takes approximately the same amount of time as the forward transaction.
Recovery from the failure of a parallel execution coordinator or parallel execution server is performed by the PMON process. If a parallel execution server or a parallel execution coordinator fails, PMON rolls back the work from that process and all other processes in the transaction roll back their changes.
Recovery from a system failure requires a new startup. Recovery is performed by the SMON process and any recovery server processes spawned by SMON. Parallel DML statements may be recovered using parallel rollback. If the initialization parameter COMPATIBLE is set to 8.1.3 or greater, Fast-Start On-Demand Rollback enables terminated transactions to be recovered, on demand, one block at a time.