Lots of people will give me crap, but the answer (IMO) to your question - is not when you are ready. Not the sex buddy part, but the dating. I wanted my W back (and still do) but after 9 months I started dating because there were no signs she was coming back.
I was careful in my dating not to hurt anyone and was honest about where I was. It made me feel better. I liked female company that wanted to be with me...:) It made me stronger, ,it helped me detach and when W found out I was dating she freaked out. She admited later that she had no right to freak out and that I had ever right to date, but she still went nuts.
Now I hoped nuts would equate with her coming back, but it didn't... well it was still fun to watch her freak and complain about the "2 weeks" of hell I put her through... yeah my love the 2 weeks of hell i put you through...