If someone has never used Gregg shorthand before, how long does it take to learn the system? On average, how long would it take to be able to write it as quickly as normal longhand? A bit faster than longhand? Full speed?
I'm currently learning Gregg shorthand, been about 2 months. If you peruse the manual, you will find the rules to be efficient and sensible. The simplicity of the individual strokes are apparent and really, not a comparison to Chinese characters.
To reach the speed of longhand, which is really at most 30 wpm (pretty slow), will maybe take few months at most depending on which version of Gregg you choose to learn. I'm already at that level with Anniversary Gregg. To reach the speed of an average typing speed (70 wpm) I would give half a year at most. Of course this would mean diligence (hour a day recommended but I've varied). To reach a sensible speed, where you can write down a speech of average speed (about 150 wpm) I would give at least a year. Of course it varies with each person and which version, but these are my approximations for the Simplified Gregg.
I would personally recommend either Simplified or Anniversary--the latter having more memory load but more efficient--to really enjoy the benefits long run.
The Wikipedia article you mentioned mentions:
We can't really put an actual time on this as that differs from person to person, learning to learning method.
If you are deciding if you want to learn it, decide on whether you really want it. I'm happy I learned Dvorak.