Once a month; every month; from month to month.

month after month: every month in an indefinite sequence; for a number of consecutive months; (hyperbolically) continuously, without respite.

Chiefly with reference to a sum of money: (so much) every month, for or in each month, monthly. Cf. per diem, adv., per annum, adv.

month by month: in each successive month; monthly without ceasing. Also: one month at a time.