For reasons stated below, this is probably not possible. Your main talking point is probably:
At 6:55AM PST Saturday April 3rd 2010:
517 of 154,637 users have 10,000 reputation
0.3% of all users have 10,000 reputation
For every 1 user with 10,000 rep, there are 299 without.
Well, given 200 reputation cap a day, we can assume that it would take 50 days to get 10,000 reputation.
50 days x 200 reputation = 10,000 reputation
Given it probably takes anywhere from 1 to 3 to 5 hours to get that 200 reputation
50 days x 1 hour = 50 hours
50 days x 3 hours = 150 hours
50 days x 5 hours = 250 hours
This means that in order to get to 10,000 reputation, you probably have spent anywhere from 50 to 250 hours on the site.
While this doesn't take into account extra reputation from bounties, accepted answers, etc, it is probably still fairly accurate because as you gain reputation, you spend extra time closing, voting, etc.
In more realistic terms, you would probably have to look at the 50 more recent users to hit 10k, and see how long they have been on the site. Throw out outliers, grab 50 more, average, repeat for a while. Unfortunately, I do not have that type of energy at 6:50 AM.
Even more unfortunately, the data dump can probably not help us here. We would have to create the entire history of each user, and see how many months they had been a member at SO when they reached 10k reputation. I believe you are unable to get that data from the dump as votes and stuff like that are anonymous-ized.