Adonis,
if you read my other posts you will see that I will never be even close to perfect.
The interesting thing is that I pretty much know what I am missing and how to correct it, but my family (wife and 4 kids), my job and the amount of other playing that I do steal the time that I would need to get the rest together - and then keep it there. No regrets though. The people that book me are generally happy with the results.
I only practice the extreme high range when I need it (maybe twice a year for the Brandenburg or similar). For big band lead playing, the F or G above high C in my normal routine is more than enough. A week or two to get used to the shallow mouthpiece (for the brighter sound) is enough.
How long it takes to get a useful double C is based on the rest of your playing. If you are doing EVERYTHING right, 6 months to a year. If you have unresolved body use or breathing issues maybe never...........
I spend about 3 months (it takes that long to get everything including articulation right) preparing for a Brandenburg, Richter or Michael Haydn. That is not only Clarke, but also Adams and just about everything else that I play on the big horn, just an octave up. At the end of the session, I always warm down with the big horn. It makes getting started the next day easier.