If using Terminal, how does one know when the process is complete? I've been staring at this screen for over 1 1/2 hours:

Understandable. Terminal is asking for your permission to change the default shell from bash to zsh.

To do that it needs your Admin password, which is the same as your usual login password assuming that your account has Administrator privileges. If you are the only user of that Mac, then you are its Administrator.

Just type the password and hit Enter. What you type is not echoed, not even with •••• characters.

The "key" icon is an indication that what you type is not echoed.

Just type the password and hit Enter or Return. Unless you do that, it will wait... forever.

If you attempt to close the Terminal window with user input still pending, it's normal for Terminal to ask if you want to terminate the processes in that window.

Or, it just hangs for some reason and never returns to the next prompt… in which case, one can either perform a ctrl-c one or more times, or ctrl-\ to really interrupt the hung process. The latter will likely spawn a dialog that one can choose to ignore.

Unless you are launching a background process, you type a command next to a prompt, and when it is done, the next prompt appears...

If you intended to include a screenshot it is not appearing for me.

To learn how to capture, edit (if necessary) and upload a screenshot to this site, refer to the Appendix in Writing an effective Apple Support Communities Question - Apple Community.