There is no clear amount of PDR that is optimal for all characters. It depends on what you need from it. Here are some examples:
32 PDR. Your firestorm barely scratched me, Diablo! Muahahaha! Fetish darts will bounce off as well.
75 PDR. Those pesky javelineers wont trouble me now, nor will the archers that often accompany them. Now I can have some peace and quiet and focus on controlling the battle with the tougher melee monsters without those interruptions.
86 PDR. Immune to the bloodstar projectiles from sirens, so I don't need to advance into an ambush just because I can't take their artillery barrage.
Or let's move to the melee fellows. The toughest ones are supposed to deal about 180 (Ventars venom lords) according to the wiki but I am doubtful - they have always been so ineffective in my experience. Nevertheless, 180. Let's cast weaken, because everyone can obtain it cheaply and from several parts of the equipment, like gloves, so it doesn't interfere with PDR equipment. Or get a wand on the weapon switch. 180 becomes 120. 120 PDR is hard to obtain but not impossible.
That does still not account for auras and other boosts but you come a long way. PDR can be the difference between being locked into hit recovery and not, which is quite important in itself. I wrote a longer answer to your similar question in my guardian thread too, if you want to compare.