the two official guides (twelfth edition and verbal supplement), taken together, provide more than enough practice.

if the student has gone through those materials and is still having considerable trouble, then one of the following three things (or more than one of them) is true:
(1) the student hasn't taken the time to learn how the problems work, and is just randomly trying to memorize things;
(2) the student doesn't understand how to read and process the passages, and is basically reading as though the passages were just factfactfactfact;
(3) the student isn't yet good enough at reading and understanding professionally written english.

notice that NONE of these three things is going to be fix-able by a greater volume of practice problems. if any of these three things is going on, additional practice problems won't fix the problem; in fact, additional practice is just going to cement the problem.
as an analogy, think of someone with a totally wrong golf swing. now, think of what will happen if this person goes out and takes 10,000 practice swings at golf balls -- the person will still have exactly the same problems, but those problems will now be so thoroughly reinforced that they will be practically impossible to fix.
the same is true for rc. in fact, i will just come out and say that no student should spend more than 15-20 hours of his or her entire life practicing specifically for gmat rc. (note that this is a lifetime total -- not monthly, not weekly, but actual lifetime.) that is plenty of time to learn how gmac writes the wording of its questions, what terms such as “primary purpose” and “inference” mean, etc.
beyond this point, gmat-specific studying is simply not going to help, and, in all probability, will make bad habits even worse and more permanent.

if someone is going to spend a large number of hours, then those hours should be spent before the person starts taking on gmat-type problems. for instance, if the student can't read english fast enough, then that's a problem that must be addressed before he/she begins to look at gmat style problems. if the student doesn't understand how to read passages for the main point, then that's a problem that must be addressed before he/she begins to look at gmat style problems. etc.