Zentyal is a great replacement for Windows domains, but in the webinterface there is currently (version 6) no option to change the password expiry settings.
To change them anyway, open a linux terminal on the Zentyal server and use the samba-tool utility
samba-tool domain passwordsettings set --max-pwd-age=0
This will turn off password aging. Change the 0 to the number of days you want to set it to expire to anything else than the default 365 days. Have look at the help page or manpage for samba-tool to see more options. If the command executes successfully, you will see the following text:
Maximum password age changed! All changes applied successfully! and you are done!