There are no official rules on this. In my opinion, it a judgment call that's best handled by your spiritual director, spiritual father, or Confessor.
If you are pursuing the spiritual life that seriously, it's important to put yourself under the spiritual care and guidance of another who can get to know you and keep you on track.
Just as the law requires a doctor's prescription for powerful drugs, so powerful penances like this, should have the prescription of a priest who has examined you and diagnosed your needs.
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