By my math, with the way the 5*s are cycling, we are officially at the point where it will take an entire year to cycle back to Kitty, for example.
Counting from 9/21/2017, based on the ‘sneak peak’ threads, it is on average about 20 days inbetween each legacy hero pair (average includes things like alliance events, special events, and 20 days includes the time they are being sold for)
so Kitty started being sold this year at 12/1/2019 for 250 CP, and will be next sold around 12/6/2020.