To be perfectly honest with you, you can approach sponsors at anytime. If you have a great site with great content and you can really promote yourself to potential sponsors they will want to work with you. I wouldn't focus as much on your traffic as on your actually dedicated readership. Meaning do you have a good deal of people that come back often and that are actively engaged when you post. Sponsors would be more pleased with the fact that you can get your dedicated followers to visit them or what not versus a ton of on hitters coming to your page but never visiting your sponsor. I don't know if you use Google Anaylitics or Quantcast but these two things can show you how many repeat visitors you get and how many page views per visitor you are receiving those are some pretty important things to look at as far as making yourself even more appealing to sponsors.