Some Windows installations have configured by default a password expiration of 42 days, if you want to disable it you might follow the next steps.

First, we need to run the lusrmgr.msc tool, this will allow us to edit local users configurations.

Next, we click on Users, and right click on the user we want to change its configuration and select Properties.

In the General tab, check the box that states Password never expires and save the changes.

Now the password will never expire. You can check again the checkbox any time if you want to enable password expiration again.