It's hard to argue that was to buy time, especially given they had spent more effort conquering their neighbors and helping the Nazis than building defenses against the them. They just wanted a larger chunk of Europe and Western Asia.

Their attempt failed because Stalin got greedy with what chunk of Europe he wanted and their poor performance against Finland convinced the Nazis to double cross them and invade.

Then?

Germany would have quite literally run out of oil (and other materials and even grain) a few months after conquering Poland. Most was imported was imported from the Americas before the war.

The French and British could have pretty much waited Germany out had Stalin decided not to bankroll the Nazis invasions of Norway and France. The allies were quite seriously considering bombing Soviet oil fields in Azerbaijan before France fell.

Presumably Stalin was hoping to prop-up Germany just long enough for them to get stuck in a protracted war in France so that he could swop in and "liberate" Europe. Unfortunately for millions it turned out to be a slight miscalculation...