How long? Try eternity. Coach Glassman once said, "There is no nirvana in CrossFit". What he meant was that no matter how much time you put into it, you'll always be challenged by it. If not, then you're not reaching out and trying all the new possibilities that CrossFit training can bring.
Of course, you can get very proficient with practice. For example, if you want to get better at double unders, then you've just got to practice -- day in and day out -- until you get them. For some, this movement comes easily, to others, it may never be something in which you excel. Is that okay? Of course. We all weren't meant to do everything in life as good as the next person. That and your body composition, injuries, physical limitations, and the like may simply preclude you from ever doing a series of double unders. Does this mean you aren't a CrossFitter? Only if you quit trying.
Another area that requires a ton of practice are the Olympic lifts. Snatching and cleaning and jerking seem easy enough until you throw some load on a bar and go for it. Yes, you can get sloppy and put weight overhead, but with time, practice, and by learning the subtle nuances of the movements, one can put significant weight overhead. How much practice? It depends, but, perhaps, Malcolm Gladwell's proverbial 10,000 hours is a good place to start. Click HERE to read a few thoughts on this concept from one of my favorite authors, Steven Pressfield.