Ashtangasamgraha 36.17

36.17

yasya tu sravati rakte mūrchā jāyate tasya vimucya yantraṃ śītasalilārdrapāṇisparśena vyajanavāyunā śrotrasukhena vacasā ca samāśvāsayannupaśamayya mūrchāṃ punaḥ srāvayet |

punarmūrchatyaparedyustryahe 'pi vā |

paraṃ tu rudhirāvasecanapramāṇaṃ prasthaḥ |

ato 'nyathā vyādhidehartubalamapekṣeta ||

If, however, while the blood is flowing, fainting occurs in a patient, one should release his ligature, revive him by touch of hands wet with cold water, by the breeze of a fan, and by speech pleasant to the ears, and having pacified the fainting, let the blood flow again.
If he faints again, [one should do it] on the next day, or even after three days.
However, the maximum measure of bloodletting is one prastha.
Other than that, one should take into consideration the disease, the body, the season, and the strength.