Vennin run starts from Lv45
Crimson Run starts from Lv51
Century Run starts from Lv55
You actually can't start running Crimson til 53 (even though you get the dungeon quests at 51). I just started running it so I know if you go into Crimson with Professor Koss' buff AND a CP Charm I... each minion you kill yields 1933 CP. Bosses 3k and change.
For Venin at 50-51, you get about 1.3k per minion if you have both Koss' buff and a CP Charm I. I'd say run Venin limited twice a day until 52. When you hit 52, go bulldoze Delphi and Blackflame and Skyreach until you hit 53. Then roll Crimson limited twice a day and rake in the glory.