My guess (with absolutely nothing to back this up) would be a 15 month celebration: October 1, 2021 - December 31, 2022.
I also have nothing to back up this guess, but, since we all know how much Disney likes money, I think there's more opportunity to make that money if they start sometime in May of 2021 and go through October 1, 2022. It gives them the whole 50th year (October 2021 to October 2022), and two summers (Jun/Jul/Aug 2021 - Jun/Jul/Aug 2022).
No, it's not too early to start thinking about this. In 1985, I went to Disneyland's 24 hour birthday party on July 17th. I was also there in 1990 for the 35th birthday, and again 5 years later for the 40th. Each time I was there, even in 1985, I wondered if I would be able to make it to the 50th. On the evening of July 16th, 2005, along with a few thousand other people, I spent the night in DCA waiting for Disneyland to open.
It might be too early to make concrete plans for this, but it's not too early to wonder what they will do for the celebration. Of course, right now everything is a guess, but it doesn't hurt to imagine what it will be like.
I don't have the emotional attachment to WDW like I do to DL, so I'm not so sure I will be willing to brave the crowds on 10/1/21, but I know there are tens of thousands of other Disney fans who will be there no matter what.
One more thing. I might not be around when Disneyland turns 100 in 2055, but, as long as I can breathe, I will be there on their 75th birthday in 2030. No, I'm not looking to make reservations this soon, but I certainly think about it every now and then.
