The months between versions so far have been 1.0->2.0: 17 months, 2.0->2.5: 12 months, 2.5->3.0: 20 months. 3.0->4.0 will probably be 60 or so months, ie 4.0 will optimistically be released early next year. An explicit goal of 4.0 is longevity; http://wiki.creativecommons.org/4.0#Goals_and_objectives doesn't say how long, but I hope for 10+ years. Each version has been prompted for a particular set of reasons.
I don't think more than a tiny improvement re DRM will be achieved, but there is still time to give feedback on this and all else, see http://wiki.creativecommons.org/4.0#Items_for_discussion
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