Bearing in mind GtA V is still going strong, with both new sales and shark cards, plus Rockstar won't want to cannibalise sales of RDR, it will be a while.
Usually they do a teser one year with a full reveal about 6 months later, releasing at least circa 18 months after the teaser. With RDR 2 in mind, the earliest you'd get the teaser would be mid 2029, putting release either later 2020 or early 2021, again at the earliest. They'd also likely try to do what they did with V, release late jn one console circle, with a later re-release on the next-gen consoles. That would fit with the early 2020s.