The Law Code of Yājñavalkya 3.1.190

3.1.190

svāśaucakālād vijñeyaṃ sparśanaṃ ca tribhāgataḥ |
śūdraviṭkṣatraviprāṇāṃ yathāśāstraṃ pracoditam ||
iti |

“Touchability is to be known as taking place after one-third of one's period of ritual impurity
for Shudras, Vaishyas, Kshatriyas, and Brahmins, as enjoined by the treatise.”