The first part of the process is to log all learning: both what you're learning and when you're learning it. This is easy for online courses (call out to Khan Academy, AI Class and Udacity) as the system has access to what content you're viewing when.
For self-directed web-based learning (i.e. reading educational webpages) it might be possible to track pageviews. For example, an add-on could be built into the browser allowing all pages in a particularly browsing session to be logged as learning. Likewise, for self-directed book-based learning it would be possible to set up a web service to log when you've read each chapter of a text book.
For school, university and other courses, logging learning activity will depend on the available systems of that institution - normally there are fairly good records of what courses people have attended when. Ideally, this information could be uploaded to an online tool, although manual entry by students into an online tool is also a possibility (particularly in the short term).
The key question for refresher content is who makes it. I would argue that it is incumbent upon any creator of educational content to create the refresher content, but this is not currently common practise.
For educational content in digital format, it may be possible to extract key items of content (e.g. definitions) automatically, particulary if there is a glossary. This may work well for the self-directed web-based learning mentioned above.
Another possibility would be an online repository where students can share refresher content for a particular course/chapter/module/book/webpage/etc. This would be combined with the online logging tools mentioned above.
There are two key types of refresher content: information (as small a unit as possible, a knol); and quizzes (to test knowledge. The information content is read/watched/heard by the student, reinforcing the information in their memory. Information content includes facts (e.g. year Shakespeare was born), definitions (e.g. definition of ignaeous rock), concepts (e.g. demand curve). Diagrams and videos make best use of our senses.
Timing of refreshers
The timing of refreshers needs to be based on research as to the optimal time post-initial-learning. Some level of user control over refresher intervals (and the option to turn off) is desirable. It might be possible to separate students using a refresher online tool into separate cohorts to actually conduct the research using the tool. Alternatively, feedback could be provided by students as to whether they considered the timing of the refresher was suitable (although this is subject to introspection illusion).
Number of refreshers
It would be theoretically possible to continue refreshers until the student dies, but this is unlikely to be necessary - my personal experience would suggest that there are some facts that are here to stay - they don't need any more refreshing.
Again, the number of refreshers needs to be determined by research, and would probably rely heavily on user feedback (the "I feel like I know this now" button).
Distribution of refreshers
The obvious medium for distributing refreshers is email (with embedded pictures, video), being free an ubiquitous. For quiz-type refreshers an email-distributed link back to an online learning site would be sensible.
Other methods (Twitter, SMS, post, podcast, RSS) could also be used. A mix of methods may be optimal to stimulate learning.
With a shared online tool, it should be possible to collate all refreshers into a single email for daily reading.
Depth and quantity of refreshers
The depth and quantity of refreshers depends on two factors: the optimal level of refreshers (research derived) based on the amount of education the student has received; and the available time of the student. I would hope that the average person could spare 10 minutes a day to make the most of the education they've received.
There are a couple of opportunities here: for existing online education services, automating refreshers is a bolt-on to the existing package; there is scope for a standalone service catering to the needs of self-directed learner; and there is a gargantuan opportunity to get traditional educational institutions converging on one tool.