I wrote some notes on this in the "Effective Implementation" book, but there is no rule or canonical convention.
To summarise some:
First of all, the most important things is that you are consistent.
General Maven convention uses '-' as the separator (commons-lang, maven-model).
If you intend for it to be used externally, remember the module = artifact ID = filename (in best practice), so it needs to be recognisable (maven-model-2.0.jar is clear, model-2.0.jar is not). In this case, you usually repeat the last element of the group ID as a mini-namespace. "Internal" modules may omit it for brevity (As the example application shows: http://github.com/brettporter/centrepoint/tree/master/centrepoint/modules/)
Aside from that, I'd keep them as short as possible.
You might draw the parallel between Java package and class naming conventions to Maven group and artifact IDs.