Brihat Parashara Hora Shastra
परायुर्द्वादशो भागस्तस्य सन्ध्या प्रकीर्तिता । तन्मिता लग्नभादीनां क्रमात् सन्ध्यादशा स्मृता ॥ २०४॥
Sandhya is the Dvadashāńś Ayurdaya of the Param Ayurdaya (maximum possible life-span). In Sandhya Dasha the Dasha of all the Rāśis from Lagn onwards is of 1/12 the years of Param Ayurdaya.
english translation
parAyurdvAdazo bhAgastasya sandhyA prakIrtitA । tanmitA lagnabhAdInAM kramAt sandhyAdazA smRtA ॥ 204॥
hk transliteration by Sanscript