The free Linux plan on CircleCI has 1,000 minutes of build time available per month: https://circleci.com/pricing
When this limit is hit, you may see the following message on builds:
Your build was not run - reason code (:org-over-credit-limit)
Your monthly cycle is based on the date the org was created on CircleCI (not necessarily 1st of the month).
You can upgrade to a paid plan to remove the build minutes limitation.
The limit is to prevent abuse of the platform so we can provide the best service for everyone.