What does Oda still have to cover:
Void Century
Giant Straw Hat/Mariejoa secret
How FI will fall because of Luffy/ Noah
What is Pluton/Uranus
Why can BB eat 2 DFs/what is up with his body/why doesn´t he sleep
Luffy´s dream
What is the One Piece
God Valley
Shank´s talk with the gorousei
Tequila Wolf purpose
Who is Im-sama and his subordinates
How were lunarians killed
Who was drinking with Crocus
The egg on Roger´s ship
Crocodile´s secret
Rocky Port Incident
Emerald City
Visit Elbaf / Loki
Unicorn/loadstar
Weevil´backstory
Rock´s backstory
Deal with Kuma´s past
Joy Boy story
Zunisha´s backstory
the D.Clan and what it means
Name of the lost kingdom
All-Blue
What destroyed Lulusia
Grand fleet incident
Logia awakening
Where is Laugh Tale
Voice of all things
Why did the robot on egghead climb the red line
Adam and Eve tree
Rioponeglyph
Black Swords
Role of Urouge
the man marked by flames/Last road poneglyph
So a lot of stuff that has to be adressed in one form or another.
My guess right now would be Egghead takes another 2-3 volumnes, after that Elbaf with maybe 5 volumnes, Loadstar and other open story parts might take 3 volumnes. The final war including fights against BB pirates and Imu/subordinates (4 volumnes alone those fights) might take 12 volumnes.
1-2 volumne for what happens after all is done and fulfilling SH crew dreams to end it all.
So 20-25 more volumnes overall would leave the series with around 1300-1350 chapters in the end.
So it going for another 5-6 years, maybe Oda combining a lot of things could bring it to 4-5 years, but can´t picture Oda not setting up the conclusions of the story properly, which takes time with his vast character line up.