Webapp - Best method to limite usage of user

Webapp - Best method to limite usage of user

I'm building a webapp where I would like to limit the user of over using
the application.
I would like to limit how many time the user send email notification each
month.
The way I was thinking of doing this is by logging every notification with
user_id date time. So before Sending a notification I will select * WHERE
user_id Between Date and Date+30. And do a validation that the number of
entry is smaller then the limit.
Is there an easy and faster way to do this. I'm a bit nervous when I will
have 1/2 millions logging entry.