About 30 minutes, so about 10 minutes more than a taxi. You could also take the X9 bus to Ernst-Reuter-Platz and then the U2 subway towards Pankow (just down the steps right next to the bus station) to Potsdamer Platz, which takes about the same time.

The thing with busses from Tegel is that when you arrive during rush-hour (especially in the early evening) the busses can be very full and, during the summer, quite hot. Wearing a suit can endanger one's life at times due to the high temperatures since a/c cannot cool down the busses when they are full on these days...

That said, the new lounge design is very nice, however the formerly attached business-center is gone (the space previously occupied by that is now part of the main lounge), and there are currently no computers or printers in the lounge. Someone with a more stylish-than-practical approach decided to replace the computers by laptops handed out by the lounge-attendands - since the lounge is not staffed all day this might pose some challenges, and as laptops are quite portable, they also may show a tendency to be away without leave. The positive change will be that the printer will be replaced by a wireless one that can be accessed from any wireless-enabled device, so printing from your own laptop/ipad/whatever will be available "soon" (ibahn has to install the printer, and there no set date for that).

Greetings - Dirk