Is there a kind of graph somewhere that shows the length of time between updates?
Not a graph, but up-to date:
0.20.7: October 25th 2020 (Odette Halloween) -> 1.5 months
0.20.5: September 7th 2020 (main story 2) -> 4 months
0.20.1: May 5th 2020 (main story) -> 5+ months
0.19.5: December 20th 2019 (Consuela) -> 2 months
0.19.1: October 19th 2019 (Eve) -> 4.5 months
0.18.5: June 4th 2019 (Jenny 2) -> 1 month
0.18.0: May 2nd 2019 (Jenny) -> 4.5 months
0.17.5: Dec 16th 2018 (Daisy) -> 1- month
0.17.0: Nov 23rd 2018 (Diane) -> 4 months
0.16.0: July 25th 2018 (Roxxy) -> 3+ months
0.15.1: April 15th 2018 (teachers) -> 4 months
0.14.5: December 19th 2017 (Debbie) -> 1.5 months
0.14.3: November 1st 2017 (Aqua) -> 0.5 months
0.14: October 13th 2017 (Mia) -> 2- months
0.13: August 21st 2017 (Erik)
The first update that is comparable to the amout of content that DC delivers nowadays has been the 0.15.1 teachers update. From then on, all major updates took around four months, followed by a minor update of around 1 month. The main story took a bit longer, but that was announced by DC before.