Every time something changes in your file
Credit scores are actually just a snapshot in time of your credit history.
The credit bureaus don’t keep your score, and don’t update it on a known schedule. Instead, every time someone pulls your score from them – they generate your score from the current data base that they have.
So the only question is how often they update their data base. Once again – there is no specific schedule.
The credit bureaus get activity information from thousands of sources. Each source has its own timing for sending the data to the credit bureaus. Once the data gets to the credit bureaus, it is processed and the database is updated.
So, If you were to check your score on a daily basis, you may see change from day to day, or no change at all for a 3 weeks.
Hope this helps.