I want to improve my chess, which is currently @ grade D-C.  I know that I need to study tactics pretty much on a daily basis, and fortunately I have some time to do that.  My question is, how long do I need to study daily (or, how many problems/patterns do I need to study) for it to be effective?

I know, the more the better, but I'd like to be able to get away with 15 minutes to half an hour.

Appreciate any comments!

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