The passwords will npt expire. You will need to create a password if you want to telnet into the device. It it always recommened to have some level of security on the device. I couldn’t have sed it better myself
Use SSH to manage devices rather than telnet. Telnet is cleartext and credentials can be captured on the wire.
There really is no “default password” on a fresh 1841, and passwords that are set have no expiration timer. You will need to set up one on your “VTY” lines so you can remotely access the device – either through telnet or SSH