Please be aware that the champions that are free in one week are not necessarily going to be free the next week. If you like a champion alot, save up your IP (or purchase RP) to get that champion so you may have him/her forever!
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All heroes will appear in rotation at some point. Not sure on probability, but I would expect champions that are harder to play well (like Karthus or Anivia) to show up less than easier Champs (Ashe). That said, I would be surprised if a champion didn't show up at least once every couple of months, Riot does a fairly good job of keeping the free selection fresh.