I believe the answer for PLD is "as little as possible". SS benefits them less than any other tank in raw numbers(see Dervy's theorycrafting blog for up to date stat weights). This is because they're still the worst off when it comes to TP preservation and Sword Oath only enhances auto-attacks, which aren't sped up by Skill Speed.

The only significant application of it I can think of is the potential of getting three combos between each reapplication of Goring Blade but napkin math indicates that you'd need a GCD in the ballpark of 2.0s to pull that off. This isn't exactly feasible XD