SailPoint Support:
I checked with the engineering team and here is what they have to say:
Inline variable is not supported in deadline for forms.
The form submission deadline is looking for an absolute value from 1-30 days or maximum of 30 days 720 hours. The Variable getting a validation error seems accurate as we would not be able to set a deadline that exceeds those documented maximums.
I ask myself how this UI change passed the QA test.