Daivajna Vallabha · Attributed to Varahamihira
Chapter 4 · Going and Coming: General Rules
सामान्यगमाऽऽगमाध्यायः
बलेन सर्वोत्तमतामुपागतो ग्रहो गृहे यावति लग्नतो भवेत् ।प्रवासतस्तत्समसंख्यमासतो निवर्तनं गन्तुरसंशयं वदेत् ॥ ८ ॥
See which planet is the strongest of all. Count its house from the rising sign. Say without doubt that the traveller returns that many months after leaving.
Simple meaning: The strongest planet's house number from the rising sign gives the months before the traveller returns.
चन्द्रतोऽथ् यदि वा विलग्नतो द्वादशद्वितयराशिगामिनः ।
Venus, Jupiter and Mercury in the 12th and 2nd from the Moon or rising sign: no going, no coming.
चरांशकस्थे किल काल एष स्थिरांशके तद्विगुणस्तु वाच्यः ।
Moving sign's navamsha: that time. Fixed: twice the time. Dual: three times the time.
यातुः पृच्छालग्नतः सप्तमस्य स्वामी यायाद्यावता वक्रभावम् ।
The traveller returns when the lord of the 7th house from the question's rising sign starts moving backward.