I see late 2019 to 2020 to be the time of the turn around, but probably not divisional winning. By then you will have Giolito, Lopez, Moncada, Kopech and Jimenez on your 25 roster almost for sure (barring crazy regression or injury), By late 2019 You should also be to the stage where Dylan Cease and Dane Dunning are getting to Chicago (or really close to it), as well as guys like Burdi, Hamilton, and Burr in the back of the pen. So at that point your pitching staff should largely be getting into place with the new faces. Less certain, but very possible is seeing someone like Seby Zavala, Alec Hansen, Tyler Johnson, and or a Jordan Stephens.
The position players are going to start hitting in more waves into the 2020 to 2021 range. Currently sitting at Birmingham you have the guys like Basabe and Collins, who could see Chicago at the end of 2019, but more realistically 2020. The guys currently at High A are much heavier in terms of talent, and are probably in the late 2020 to 2021 crowd. That is where you get into Robert, Rutherford, Luis Gonzalez, Micker, and Madrigal currently. I get the feeling that Robert, Gonzalez, and Madrigal could do a quicker move, with the more raw guys taking a bit longer. The Kannapolis team has a crowd of guys behind them that will factor more into the 2021 to 22 crowd.
That is a long way of saying that if you actually sketch it out on paper, it is still going to be a while if you have reasonable expectations for players.