I used this "rule" and am using it now to get all the 90 cp heroes.
First, we'll plan a target of earning CP per day. 15 cp is good target. My target is actually 20cp/day.
I'll take 15cp. Earn 15 cp every day. Hence you get 90 cp for 6 days, and you should earn 15 cp on Sundays too.
Every 6 days, you should get a 90cp hero.
This means you will should get all the 90cp heroes in 102 days, or 25 weeks and two days, or 6 months and a week.
You should earn extra CP if possible too.
Best of luck!