Join Date: Feb 1999
Location: San Jose, California, USA
Programs: AS Plat, UA MM, AA MM, IC Plat, Marriott Gold, Hilton Gold, Hyatt Globalist
Posts: 3,162
As one data point, here are some notes I kept from my last visit to Jakarta in January 2019:
3:25 Arrive into T3 terminal
3:35 Arrive SkyTrain
3:43 SkyTrain departs T3
3:54 SkyTrain arrives Stasiun Bandara
4:20 Train departs Stasiun Bandara
5:06 Train arrives Stasiun BNI City
5:15 Grab Car arrives
5:45 Arrive Aloft Hotel
10:43 leave aloft hotel in grab car
10:54 arrive BNI city
11:00 complete ticket purchase
11:21 train departs
12:06 Train arrives
12:10 arrive SkyTrain
12:19 SkyTrain departs for T2
12:21 arrive T2 SkyTrain
Xxxx Arrive CX check-in
I wasn't meticulous with my timings (I put a placeholder for my arrival at the check-in desk but forgot to fill it in). Regardless, you can see that it took me over 2 hours to get from the airport to my hotel in central Jakarta, and a little less than 2 hours to get back the next day. Unfortunately, a 7-hour layover does not guarantee enough time for a visit to central Jakarta unless there are 1-2 specific things you know that you want to do (visit a site, eat at a restaurant, or buy a gift) and are happy doing only those 1-2 things.