Frequency/quantity in a/one month

When describing repeated actions, you can use in if you're referring to a specific time period.

He smoked five cigarettes in one day.

I did it five times in a single year.

This is often used to express that the particular time period was exceptional, although it could just be describing an exemplary period.

But if you're describing typical behavior or average frequency, you don't use in.

He smokes five cigarettes a day.

I do it five times every year.