How often the manuals are updated varies from producer to producer, so this question is very specific to the publisher. I presume from your description that you are referring to the Oxford Jeppesen manuals. I can answer this because I happen to be an Oxford instructor.
New editions are produced as and when there are sufficient changes to justify it - which is a bit arbitrary, but should work out at about once a year. The Oxford Jeppesen manuals are now Edition 2, published fairly recently. No decision has been made on a publication date for Edition 3, but I would expect it to be in about 10 -12 months.
However, essential updates, especially any errors noted, are available on Oxford's website.