First day of use: 11th Dec 2014
First missed day: 19th Jun 2017 (921 days later)
First freeze use: 20th Oct 2020 (2,140 days after first day of use)
The missed days were down to (IIRC) time zone crossing and patchy internet, I can't recall if Duolingo let you off here or if there was leeway in the streak calculation. Those could also be Duolingo being down for maintenance.I think when I started using the freezes was when the value of the app had dropped for me. 2,140 days is a pretty good run.
FWIW: living in a French speaking country has provided the most value. Duo helped initially with vocab building. I find/found the lack of colloquialisms one of the biggest problems with it.