your calculation is wrong, -1 d8d9dadb -2 808182...etc...bebf in combination with ?1?2..etc does not mean 68^n possibilities because you can't just add ?1 and ?2, you are multiplying. For every char in ?1 you are doing all the chars in ?2.
so your command is actually 256^n possibilities. With 6 chars at ~11500 MH/s you will need about 7 hours.
so your command is actually 256^n possibilities. With 6 chars at ~11500 MH/s you will need about 7 hours.