I have played much more than that. I've done n backing over the course of one year, although not consistently every day, and not every day as much as 20 minutes. But in the beginning I had that schedule.
Whether it is worth it depends on what your goals are.
If your aim is to improve fluid intelligence, it will not necessarily do that (some studies report transfer to fluid intelligence [as tested with Raven's Progressive Matrices type IQ tests], others do not, so the jury is still out on why some tests report transfer and others do not, the latest theory I read was that the aspect of n backing that may improve fluid intelligence is the prolonged exercising of your executive functions, something that n-backing does if you have not formulated clear strategies for it.
You have very little to lose. I would recommend that you take a Raven's Test online before commencing your training (stupidly, I did not do that so I don't know whether I experience any transfer or not, but even if I had, I also made additional changes to my lifestyle during the same period that might have had an effect [fish oil, piracetam, gotu kola, curcumin supplementation/cardio exercise/brainwave entrainment].
Anyway, to keep stressing executive function, my theory is that changing aspects of the working memory training often could be beneficial. Brain Workshop and other n-back implementations allow you to do that. You can also practice other types of exercises that stress your working memory (mental math and complex working memory at brainscale.org), (forward/backward digit span at cognitivefun.net) monkey ladder, spatial search and digit span at cambridgebrainsciences.com .
I have experienced a boost in working memory capacity. My backward and forward digit span are both at 10-11 now, while previously I would say they were at around 7 or 8. The cognifit website lists 2 of my cognitive abilities as being at 800 (max): working memory and focus attention.
Edited by Godof Smallthings, 30 May 2013 - 08:43 AM.