One month is 30.436875 days on this site — the Gregorian year divided by twelve. No actual calendar month has that length, which is the whole point: there is no such thing as a standard month, so any conversion has to pick an average and say so.
About the units
Calendar months run from 28 to 31 days, and the pattern is a historical accident rather than a design. The Roman calendar originally had ten months starting in March, which is why September, October, November and December carry the Latin numerals for seven through ten while sitting in positions nine through twelve. January and February were added later, and July and August were renamed for Julius and Augustus Caesar. The synodic month — the actual period between new moons — is 29.53 days, and it does not divide the year evenly either, which is why lunar calendars drift against the seasons and lunisolar ones need intercalary months.
The exact factor
The value used here is 365.2425 ÷ 12 = 30.436875 days, the mean Gregorian month. Other conventions exist and give different answers: 30 days is the commercial standard in some financial contracts, 30.4375 is the mean Julian month, and 29.530589 is the synodic lunar month. A calculation spanning several months should state which convention it uses, because over a year the difference between 30 and 30.44 days per month accumulates to more than five days.
Where you meet this conversion
Subscription and rental billing, where a monthly charge has to be prorated across a partial period. Medication schedules, loan amortisation and payroll accrual all need a day count for a month, and the choice is usually specified in the contract rather than left to arithmetic. Statistics that report monthly rates — rainfall, births, sales — are often normalised to a standard month length so that February and March can be compared fairly. Astronomy and calendrics use the synodic month instead, since that is the one tied to an actual physical cycle.