The Soviet/Warsaw Pact 'window of opportunity' was probably the late 70s/early 80s, IMO.
Earlier on the cold war the NATO technical superiority (especially in nuclear weapons, we know now) was probably enough to deter the Pact, although the West didn't know it at the time.
But by the late 70s the oil shock had badly damamged Western economies and defence spending had reduced, plus the US Armed Forces was in the Vietnam after-shock and was something of a hollow shell. (Drugs issues etc)
Assuming a conventional offensive into Western Europe - the 'usual scenario' - it's hard to imagine the NATO forces stopping GSFG. The Pact would probably have gone chenical early too, which would have pretty much devastated the FRG.
Chances have to be that the Russians would have got to somewhere near the lower Rhine before running out of steam, at least. While the Russian CatII and CatIII divisions were pretty much cannon fodder, with obsolescent equipment, at least they HAD a second wave - NATO was pretty much a one shot deal, once the REFORGER units were in theatre.
The question is whether the Pact would have stopped at the Rhine. The question of French nuclear retaliation would come into play quite quickly, had they pushed on to Paris, say. Similar considerations apply to an attempt to take the UK.
Of course, one has to wonder WHY they would have attacked. After all, there wouldnt have been much left to loot. I guess its a good thing they never thought of a good reason.