In Cents Business Manager, go to Admin > Promotions and click the promo code. (Setting up a new code? These options are part of creating it.)

In the promotion details, update the start date. It can't be in the past.

Add an end date if the offer should stop on a certain day. Leave it empty to keep the code running until you turn it off.

Choose the days of the week the code works, or keep All Days Selected.

In the same promotion details, open the usage limit (it shows No limits by default).

Set how many orders each customer can use the code on. You can also limit it to a customer's first orders only.

A limit of 0 means unlimited.

Applies to: the entire order, or only specific products and services.

Minimum requirement: a minimum order amount or number of items before the code works.

Customers: any customer, or only specific customer groups.

For the full setup, see How to Create a Promotional Code (Percent or Dollar Amount).

Codes only work between the start and end dates, on the days you choose.

A customer who reaches the usage limit can't apply the code again. The current order doesn't count toward the limit.

Dates are by day. There's no start or end time.

A code says it's expired or hasn't started.
Check the start and end dates in the promotion details.

A code doesn't work on certain days.
Check which days of the week are selected.

A customer says they can't use the code again.
They've reached the usage limit. Raise the limit or set it to 0 for unlimited.