I was once told "There is an "official" length of time that you have to comply with to work as a dp. But really and truthfully it is up to you how long it takes."
From general consensus, the "official" length in the UK seems to be 10-15years to become a dp. Going through all the steps.
As someone that has a natural tendency to break the rules, I never settled for that, I spent little time as an AC. Its not what I enjoyed doing.
I worked very very hard to get broadcast work as my bread-and-butter. This then allowed me to work for free on low budget films.
I'm not sure whether what I've done so far would've been possible 25 years ago, as the literature and information that we have at such easy disposal these days, wasn't around then.
Coupled with each individuals desire to achieve, its authors like Brian and David(and forums like this one), who have shared what they have learnt, that has made it and will make it possible for ppl to make their own choices, instead of being pushed into the process set by the establishment.