Do the devas have finite lifespans? Or do they live perpetually? Please cite scholars, not just scripture alone.
Bhagavad Gita 9.21
ते तं भुक्त्वा स्वर्गलोकं विशालं क्षीणे पुण्ये मर्त्यलोकं विशन्ति | एवं त्रयीधर्ममनुप्रपन्ना गतागतं कामकामा लभन्ते
When they have enjoyed the vast pleasures of heaven, the stock of their merits being exhausted, they return to the earthly plane.
(English Translation Swami Mukundananda)
Srimad Bhaagavad Maha Puran 11.10.26 - Sri. Ved Vyas
tāvat pramodate swarge yāvat puṇyaṁ samāpyate kṣhīṇa puṇyaḥ patatyarvāganichchhan kāla-chālitaḥ
“Residents of heaven enjoy the celestial delights until their merits have been exhausted. Then they are reluctantly forced to fall back to the lower abodes by the passage of time.”
Srimad Bhaagavad Maha Puran(Dhuruva Stuti) 4.9.10 - Sri. Ved Vyas
sā brahmaṇi sva-mahimany api nātha mā bhūt kiṁ tv antakāsi-lulitāt patatāṁ vimānāt
what to speak of the temporary blissfulness of elevating oneself to the heavenly planets, which is ended by the separating sword of time? Although one may be elevated to the heavenly planets, he falls down in due course of time.(source)
Vivekachudamani verse 6 -Sri Shankaracharya
वदन्तु शास्त्राणि यजन्तु देवान् कुर्वन्तु कर्माणि भजन्तु देवताः । आत्मैक्यबोधेन विनापि मुक्तिः न सिध्यति ब्रह्मशतान्तरेऽपि
There is no Liberation without the realisation of one’s identity with the Ātman, no, not even in the lifetime of a hundred Brahmas put together.
No abode is absolutely permanent. All abodes are temporary, until Self-realization.