Brihat Samhita
ऋज्वीअतिवक्रावक्रा विकला च मतेन देवलस्यएताः । पञ्चचतुर्द्व्येकाहा ऋज्व्यादीनां षडभ्यस्ताः ॥ ७-१५ ॥
According to Devata, the effects of the Ṛjvya (direct), the Ativakra (over retrograde), the Vakra (retrograde) and the Vikala (of irregular rate) motions of Mercury will last respectively for 30, 24, 12 and 6 days.
english translation
RjvIativakrAvakrA vikalA ca matena devalasyaetAH । paJcacaturdvyekAhA RjvyAdInAM SaDabhyastAH ॥ 7-15 ॥
hk transliteration by Sanscript