Look at this American conversation
Isabel: Hi Vicky, this is Isabel. I'm the company's travel coordinator.
Vicky: Hi Isabel. Is there a problem?
Isabel: No. I see that you take quite a few business trips a month.
Vicky: Yes, I do.
Isabel: I noticed that you are not signed up for frequent flier miles.
Vicky: No, I haven't signed up yet.
Isabel: Please sign up before your next trip. You can earn miles and get a free trip anywhere in the country.
Vicky: I didn't know that.
Isabel: It's easy to sign up. You can do it online.
Vicky: I will do that today since I have another business trip at the end of the week.
What does "a month" mean in "I see that you take quite a few business trips a month."?
I know that American people often omit prepositions before Mondays, mornings.
Example "Do you work Saturdays?"="Do you work on Saturdays?"
I am not sure whether they omit "in" in "in a month".