This is a know how KBA on the field "Validity(Days)" field, which plays an important part in deciding the number of days the OAuth would be valid for.
Question: Can we leave the field blank while generating the X.509 certificate?
Answer: Yes, when the field is left blank system defaults the validity to 365 days
Question: Can we use our own period of validity?
Answer: Yes, Successfactors provides the flexibility to give the number of days as per the need to suit your requirement
Question: Can we generate our own X.509 certificate and use it?
Answer:Yes, this flexibility is provided in Successfactors
Validity of keypair generated using OAuth
Validity of the X.509 certificate uploaded to Successfactors