According to the Aeroflot website, they currently fly out of Schönefeld. I strongly suspect they will continue to do that until the new airport opens.

The train station at Schönefeld is about 300 meters from the terminal building. You shouldn't need more than 10 minutes to get there.

There used to be many direct trains from Schönefeld to the main station (Hauptbahnhof), but there don't seem to be any now. Most train journesy require one connection. Even with this connection, the time needed to get from Schönefeld to the Hauptbahnhof is approx. 45 minutes. Trains run about every 10 minutes. For more details on this, check out the railroad website, http://www.bahn.de/i/view/overseas/en/index.shtml.

I am not sure why Fanjet warned against the S-Bahn trains. They are indeed local trains, stopping at every station, but because of the way the schedule is now set up, getting a normal train won't save you more than 10 minutes. And the regular trains don't run nearly as often as the S-Bahn.

If you post your details: arrival date, time, and final destination, we could better estimate your chances!