It was done in stages. First iteration of "wall" was just stacks of razor wire dragged across border line (You can see how it looked like in this famous photo of East German soldier letting young boy, who found himself on wrong side of fence, separated from parents, sneak by ) - you can "build" such "wall" pretty much as fast as your soldiers can run with stack of wire on their back. "Final" version of wall that you think of (and probably saw being dismantled in 1991), with concrete elements, watchtowers etc. took five years to build (1975-1980).