When your credits reset
Plan credits refill monthly on monthly and yearly plans. Check your profile menu for your next reset date. Unused credits roll over for one month.
Your plan credits refill once a month, including on yearly plans. Click your profile picture to see your next reset date: "Monthly credits reset ...".
Rollover
Credits you did not use stay for one more month, up to one month's allowance. If your plan gives 500 Building credits and you have 200 left on reset day, you start the new month with 700. The month after, any of those old 200 that are still unused expire. The Usage page lists this as "Unused credits expired".
After an upgrade
When you upgrade a monthly plan, you pay right away and your billing month restarts that day. The full new allowance is added to what you had left. Your reset date moves to one month from the upgrade.
Bonus credits
Credits from converted Rewards points do not expire at the monthly reset. The Usage page shows them as "bonus credits". If you cancel your plan, they are removed with your other credits.
Common problems
My credits did not reset on the 1st
Credits do not always reset on the first of the month. Check your next reset date in your profile menu.