I created the RunPee app: tells you the best time to run and pee during a movie without missing the best scenes.
It was created with Adobe AIR back in 2009. I maintained the code and added features for the next 11 years. So I knew this code like the back of my hand.
In 2020 I decided to rebuild the app using Google Flutter. I spent about 6 weeks getting acquainted with Flutter and then started rebuilding. I guessed that it would take 9-12 months. The only real uncertainty was my unfamiliarity with Flutter because I knew exactly what the features were and how they needed to be built.
About 3 months into the project I started thinking that I was actually going to finish way ahead of time. I even thought I might make a November launch date, ahead of the Christmas movie season.
But then testing, and the last few "minor" features ended up dragging on and wouldn't you know it. It took 10 months from beginning to end.
I guess the real point to this longer than necessary story is that during a project your sense of nearness to completion will fluctuate. But still, it will take as long as it takes. Just put your head down and do the work.