If you want to go back to the original concept of Shenmue, when it was called Virtua Fighter RPG and starring Akira. I think the original plan was to follow the life of Akira from being a teenager to his first tournament in Virtua Fighter 1. I think the idea was to start Akrira as a teenager in 1986 and end the series sometimes in the early 1990's when he is in his 20's. The idea was to simulate 6 or 7 years of uninterrupted game time as Akira. As opposed to just a simple origins story as the character.
But the Virtua Fighter connection was thrown out, and replaced with Ryo and an original storyline.
I don't know how long Suzuki planned on making the Ryo storyline go on for. Shenmue 1's start date is December 1st 1986, and it's end date is April 15th 1987, which is 135 playable days total. Shenmue II's default start date is February 23rd 1987, and its end date is June 31st 1987, which is 157 playable days total. Though Shenmue II's start date can change depending on player save data carry over from the first game. Shenmue II can start as early as January 16th 1987, if you speedrun through Shenmue 1. The latest date Shenmue Ii can start on is April 19th 1987, four days after the events of the end date in Shenmue 1. The boat trip takes four days total from Japan to China.
Shenmue III does not carry over from Shenmue II sadly, given the different engine and technology being used. So it will have a fixed start date. My guess is probably July 1987? The game probably ends in 1988. Shenmue IV would probably go from 1988 to 1989. Or something?