There are a lot of different answers in this thread. Not all of them are wrong, but they don't fully answer the question.
The amount of PUR you need depends on your build and whether you play solo or multiplayer. In general, you don't want more than you need, because the slot could be used for more dps/ehp. With a solo bats build 6 yards is enough (best on boots, pants, or crafted gear). GI has an inherent 8 yard radius so this will give you 14 yards. However, basically any other build requires more, including bears, spirit barrage, 0 dogs, AC, etc. I would suggest 20 PUR for these builds. I use 6 yards when using bats and it's plenty.
In multiplayer it is usually best to have more than 6 yards. Monsters will be dying all over the screen, so +14 yards is advisable.
However, if you're using the build currently showing in your profile then PUR is not particularly relevant. PUR is used by WDs to proc GI and reduce cooldowns. If you aren't using GI, PUR won't help you all that much. You should aim to include GI in your build. If you plan on using a 1h, I think Thing of the Deep is best option for your gear level. If you plan on using a Skorn (advisable if you want to do mp10) you'll have more difficulty getting PUR.
Looking at your current skill set it seems you are having troubles with mana. You're using both BR and SA with a bats build, while having a low attack speed. This should give you far more mana than you need...do you really need both mana passives? Are you recasting CoB often in battles, or channeling for long periods? I think if you equipped a Thing of the Deep you could drop BR for GI. You will lose a small amount of dps, but you'll gain mana, allowing you to use a much more effective skill setup (ie. GI). This will also allow you to channel bats longer and regain mana from all the monsters that run in. You'll also be able to cast spirit walk a lot more, which adds more mana regen. And the best part is that GI and GF synergise very well when played correctly. Most WDs gear themselves to allow for this combo, rather than adapting skill sets to their current gear. Try it out and see how it goes :)