It should not be forgotten that this would have been part of a much larger european campaign so the West Berlin take over would need to be done to neutralise the NATO assets and protect USSR/GDR supply lines from attack deep inside their own territory.
In terms of how long the major factors would be
- Who started the shooting and how and where it started. If NATO for some reason decided that they wanted to liberate GDR, then I'd give the Berlin based NATO forces good odds for staying active until reached by forces pushing from the West. If it was the Warsaw Pact forces doing the invading then good odds they'd be overrun within a day or two of the invasion starting.
- What happens with the main front of the campaign on the West/East German borders. If that front was going well for the Warsaw Pact forces, then I'd give the estimates by the GDR experts some credence, if however things were turning in favour of NATO, then the likelyhood of survival would increase with every mile of ground taken by NATO.
Either way the result would have been ugly for both sides.