Given Rockstar's seeming track record of about 5 years between GTA 5 (for PS3/XB360) to RDR2 (2018 for XBO/PS4), there is a good amount of sense to the project taking about 5-6 year's of development.
If they've been going at things similar to GTAV for the announcement/news cycle, than I'd say we are a way's off until we get information.