How to choose the best duration to churn? E.g., this customer will churn within a month

Question

The result would be something similar to:

• Customer xxx will churn within n months/weeks.

My data:

1. Business data about each client [company type, country, contract type, etc.]
2. Activity data [when they did some activities related to the service...].
3. A column that shows if they are churned or if they are still active.

My questions are:

a) How can I calculate a good duration for churn? E.g., is a week before they churn is good, or should it be longer? Is there a formula to calculate this?

b) How do I prepare my data for the chosen duration? E.g., if we want a month of churn [probability of the customer xxx churn within the next month], should I use the activity data up until last month? In other words, should I exclude last month's activity data in my model?

I'm going to use a random forest model for this task.