Hi Pedro,
this is not exactly what you were asking for, however you might find it usefull.
1. First setup SLA for your process ID - go to SPRO - GRC/Access Control / Workflow for access control / Provisioning/ Maintain Service Level Agreement to for example 15 days. This will not appear on this filed - however if the field will not be filled out - default will be 15 days.
2. Hide this field 'due date' using application config from access request form.
3. Every request will now have a deadline for 15 days or whatever you setup in 1.
Regards,
Filip