Whilst not a definitive guide, sometimes looking at history we can get a glimpse of when a Rails minor release is likely to see the light of day.
I went through each major release of Rails and worked out the number of days between its release date and that of its next minor release.
Rails 3.0 - August 29, 2010
Rails 3.1 - August 31, 2011 (370 days)
Rails 4.0 - June 25, 2013
Rails 4.1 - April 8, 2014 (318 days)
Rails 5.0 - June 30, 2016
Rails 5.1 - April 27, 2017 (332 days)
So the average days between major and the first minor release is 340 days.
Rails 6.0 was released on August 16, 2019.
All things being equal, based on the average, we should see Rails 6.1 on or around July 21, 2020*. There will be release candidates and betas in the weeks prior.
Test out the master branch of Rails in your apps. Report bugs. Help out on the Rails issue tracker.
We are ~6months from 6.1 being released (based on my super scientific crystal ball). If you think your app is going to take 6+ months to develop then yes, I would start writing it using 6.1 with the benefits that come from the fixes already there and the new features.
That said, Basecamp, Shopify and Github are all comfortable using Rails master in their apps today!
The more master gets used the better the next point release will be for everyone.
*Pure semi-scientific speculation