I'm trying to understand, the Hindu timescales, but unable to understand if Vedas are revealed in every mahayuga or manuvantara, does any purana provide info as to when were Vedas revealed?
According to the ParAshara Smriti (PS) the Vedas are recalled by Lord BrahmA at the beginning of each Kalpa.
Na kashchitvedakartA cha vedam smritvA chaturmukham |
Tathaiva dharmAn smarati manuh kalpAntareantare ||
The author of the Veda there is none ; (he) the fourfaced (God), at each succeeding revolution of a Kalpa, recalls to mind the Veda ; and so does Manu remember the law (at each succeeding revolution of a Kalpa)
According to Shvetashvatar Upanishad, Brahman delivers the Vedas to Brahma while creation:
यो ब्रह्माणं विदधाति पूर्वं यो वै वेदांश्च प्रहिणोति तस्मै ।
तं ह देवं आत्मबुद्धिप्रकाशं मुमुक्षुर्वै शरणमहं प्रपद्ये ॥ १८॥
Seeking liberation, I take refuge in the Lord, who projected Brahma (the Creator) and delivered to him the Vedas, and who is the revealer of self-knowledge.
So, the answer should be "at the beginning of each new Kalpa".
The Vedas are revealed to human civilization only at the beginning of every Treta yuga.
11.17.11: In Satya-yuga the undivided Veda is expressed by the syllable om, and I am the only object of mental activities. I become manifest as the four-legged bull of religion, and thus the inhabitants of Satya-yuga, fixed in austerity and free from all sins, worship Me as Lord Haṁsa.
11.17.12: O greatly fortunate one, at the beginning of Tretā-yuga Vedic knowledge appeared from My heart, which is the abode of the air of life, in three divisions — as Ṛg, Sāma and Yajur. Then from that knowledge I appeared as threefold sacrifice.