The Spartan-III operations PROMETHEUS and TORPEDO prevented the Covenant from making [i]major[/i] incursions into UNSC space - the Covenant were setting up shipyards in areas that would take under two weeks to reach with slipspace travel. Kurt literally says that if the Covenant had not had their presence removed from that area of space then they would have steamrolled every Orion-side colony. In addition to the loss of invaluable strategically located bases of operation, the Covenant lost a [i]wealth[/i] of essential resources for their fleets, along with thousands/tens of thousands of soldiers during PROMETHEUS and TORPEDO. These two victories [i]alone[/i] are a big part of the reason why humanity managed to last from the 2530s to the 50s. We can extrapolate the amount of time that the Spartan-IIIs of Alpha and Beta company bought humanity from the years in which PROMETHEUS and TORPEDO took place. The former took place in 2537, the latter in 2545. Alpha effectively bought us 8 years, and Beta got us 7.