When credits are allocated to a candidate, they remain available for up to 10 days. After this, the invite will expire, and the candidate can no longer use the credit.

If a candidate logs in and tries to take a test, the system will say that they do not have credits allocated to them. No credits will be deducted from the account holder - the expiry is just a cut-off to prevent credits being used a long time after being allocated when you might assume the candidate was not going to use them.