It varies tremendously from person to person.
For me, the decision to get a divorce took ~4 years. The last year was mostly me convincing myself that an unattractive computer nerd could find someone else even remotely as good as the ex (looks exactly like Jennifer Love Hewitt) and had a near perfect personality match to me. I thought it was impossible. So I stretched out a failed relationship for years longer than I should.
By the time I finally convinced myself to take that divorce leap, I was ready to get rid of my ex. I had no feelings at all for her. Heck, I was pretty much over with the feelings of anger too. I filed for divorce, and met my current GF within a week. I wasn't after a rebound either. However, I didn't let myself date. I wanted the ex to first move out and the divorce to be mostly finalized. I met my current GF again 2 months later (this was the next time we saw each other) and asked her out. We started dating a couple weeks before the divorce was finalized, but my GF was cool with that. Things have been pefect for a year now.
So basically, I was ready immediately but didn't let myself date for 2 months. I would really worry about someone if he/she still had feelings for the ex or who was still in the anger period, or who is looking for a rebound. Those people may need another year.