when i was pulling cable on a pilot for HBO, i asked the DP this same question, and he told me that you should really get your feet wet on as many sets you can as a camera assistant (i guess a focus puller or an operator), but he also said that there is no real "chart" to follow.
a lot of DPs start off at the bottom, but a lot of them work on indie projects and get noticed that way; if you meet a producer with enough pull, he can get you a union card without all that fuss (minimum hours, the catch-22 of working on union projects versus being in the union, or not being in the union and having a mentor that will pay you so that you can reach your goals--then join the union)
i can't say for sure, i've only been a lowly runner and cable puller, i just finished my MFA and i'm taking any cabling / camera assistant / clapper job that comes my way.
my father said that's important to develop a body of work, know what good looks like, know what poop looks like, and have the wisdom to understand how a certain project under your belt might damage your reputation in this business. *shrug* i'm just a kid, so i'm taking it day by day, but looking at the careers of top DPs working on big budget projects, it looks like a best bet to keep your nose on the grind stone 16 hours a day, and learn everything you can about it, and then start getting involved with talented people that will push the limits of your knowledge and hopefully help you evolve artistically.