The Dual N-Back FAQ is a great resource compiling tons of research and advice on using the dual n-back test for improving working memory and/or general intelligence.
SHOULD I DO MULTIPLE DAILY SESSIONS, OR JUST ONE?
Most users seem to go for one long N-back session, pointing out that exercises one’s focus. Others do one session in the morning and one in the evening so they can focus better on each one. There is some scientific support for the idea that evening sessions are better than morning sessions, though; see Kuriyama 2008 on how practice before bedtime was more effective than after waking up.
If you break up sessions into more than 2, you’re probably wasting time due to overhead, and may not be getting enough exercise in each session to really strain yourself like you need to.
And as for frequency/spacing:
This study compared a high intensity working memory training (45 minutes, 4 times per week for 4 weeks) with a distributed training (45 minutes, 2 times per week for 8 weeks) in middle-aged, healthy adults…Our results indicate that the distributed training led to increased performance in all cognitive domains when compared to the high intensity training and the control group without training. The most significant differences revealed by interaction contrasts were found for verbal and visual working memory, verbal short-term memory and mental speed.
It also includes a meta-analysis of studies critical of n-back and found evidence for only one flaw: use of passive control groups, which accounts for about half of the improvement in IQ in some studies.
I found it via Brain Workshop, an opens source n-back application for Windows, OSX and Linux.
Previously: History of the n-back training exercise.