After a word denoting time, or length, the affix of the 2nd case is used, when denoting a full duration.,
मासमधीते 'he reads for a month,' 'he studies full one month,' मासं कल्याणी 'prosperous during the month' (uninterruptedly), संवत्सरं पुष्पाः 'flowers during the year continually,' क्रोशं कुटिला नदी 'the river winding for one kos without any break,' क्रोशं पर्वतः 'the hill through one full kos.' सभा वैश्रवणी राजन् शतयोजनमायता 'O king the hall of Visravana is 100 yojanas in length.'
The word अत्यन्तसंयोग or 'full continuity' means the complete relation of time or space with its action, attribute or substance.
Why do we say 'atyanta-sanyoga or complete continuity'? Observe मासस्य द्विरधीते or क्रोशस्यैकदेशे पर्वतः ॥,