People are starting to beat Borderlands 3 already, and the first reports about how long it takes to beat the game is already in. So far it looks like it will take a regular player about 20-25 hours to beat the main game. This is if they focus mainly on the main story line, and don't spend a lot of time doing side missions, do crew challenges, or just run around doing nothing.
It's also a lot faster to complete the game if you are a group of four people who play it together. This reduces the total time estimated to beat Borderlands 3 by many hours.
So, in summary, expect to spend just shy of 20 hours if you play with three friends, or just over 25 hours if you play solo.