Just one quick question...
How long would you think it would take to be able to have a simple conversation in Japanese if I begin to learn Japanese with Duolingo 20 minutes a day?
Honestly, if your only learning resource for Japanese is DuoLingo, I doubt you would ever reach that level of profiency.
Unless you count a simple exchange of greetings or asking someone a basic question to be a "simple conversation".
And you can reach that level much faster using a phrase book.
DuoLingo Japanese provides a foundation, but you will need to expand your studies beyond this app to reach conversational level.