Every beginner underestimate how much salesmanship it takes to run a successful freelancer career.
I'm comfortable doing sales, but it's maintaining that balance that I find difficult – if I spend too much time pitching I make no money as I have no employees to deliver the work and if I spend too much time working I get stuck in a cycle of feast and famine.
Similarly, if you want to spend all your time programming, a true, short term consulting business isn't going to get you there.
I'd assume that (if all goes well) you'll bill 1000 hours a year (about 50% utilization with the balance going to sales, marketing, accounting, updating software, keeping up to date with the industry, etc). It's conservative (assuming you sell enough to be busy 40+ hrs a week every week), but it's better than assuming you'll bill 2000 hrs (40 hrs x 50 weeks) and finding you have to work 70 hours every week just to get by.