How long does it take for a skill learned offline to update on the website?
I learned a skill in the car, how long before it is updated and appears gold on the website?
As soon as the app shows the streak/lingots syncronize in the right above corner on the Android app. This means it syncronized, especially when the streak circle is full, so the tree/skills should be updated too.
Offline: My opinion: Don't do it!
The offline app functionality looks quite buggy to me, even on the newest V3.42.1: https://www.duolingo.com/comment/20610216
My last test was 5 offline lessons for ONE skill, not TWO as my two tests (2+1/2+1 or 5+4) before.
The Android app did NOT start anymore after I successfully closed the app normally and the tree could not be synced anymore successfully (self termination of the app).
With very much luck it synchronized later (many many starts) only 1 of 5 lessons, and passing midnight 00:00 o' clock it suddenly could start the app and sync the rest of the 4 of 5 lessons the next day.
For all my other 2-3 tests this never happend that the online sync worked days after.
Doing offline lessons IMHO means 99,99% (how it is programmed right now):
To maybe (probably!) loose all saved lesson progress, no matter if one or two skills!
Data backups do not work too!
My tests have been made on BlueStacks on Windows, not an Android mobile phone....