This really depends on the nation you are choosing. The maximum I ever set any provincial defense at is 21. The expenditure of money is not worth it past this point (and even lower for some nations) because you can field much better armies than PD would be able to create. If you have a problem with players snatching your rearmost provinces by either Stealth or spells, I usually change my lines so that patrollers make it harder, or reply in kind with spells to retake them. Alternatively you can have a 'mop up' army that can take care of anything the PD can't handle.