Kautilya's Arthashastra 9.2.9

9.2.9

sainyam anekam anekastham uktam anuktaṃ vā vilopārthaṃ yad uttiṣṭhati tad autsāhikaṃ - abhaktavetanaṃ vilopaviṣṭipratāpakaraṃ bhedyaṃ pareṣām, abhedyaṃ tulyadeśajātiśilpaprāyaṃ saṃhataṃ mahat // [iti balopādānakālāh]

That army which is vast and is composed of various kinds of men and is so enthusiastic as to rise even without provision and wages for plunder when told or untold; that which is capable of applying its own remedies against unfavourable rains; that which can be disbanded and which is invincible for enemies; and that, of which all the men are of the same country, same caste, and same training, is (to be considered as) a compact body of vast power.

Such are the periods of time for recruiting the army.