You can download a test application with one table and one form, which uses the script, from here: https://github.com/OlegKi/Show-Hours-Only-in-Duration

You cannot use getControl with duration field.

I'm not sure what you mean under "You cannot use getControl with duration field". Do you mean that it's not allowed? I posted fragment from the code of application, which is certified by ServiceNow and which is available on https://store.servicenow.com/.

Thanks everyone.  So there is no out-of-the-box solution other than high-level customization through the use of DOM manipulation which could be prone to errors during new releases?

Was anyone able to hide the seconds field box on the portal?