We would expect any oak timber to last anything from 100 to 500 years providing it not subjected to infestations or excessive exposure to water.
The example we have used is an air-dried beam which means it has been left to dry in the open for several years and therefore has a low moisture content of around 30%. This will reduce the amount of movement in the replaced beam and subsequently will lessen the interference with the oak beams that are already in place.
In a new build we would use fresh sawn green oak which would have a moisture content of anything between 60 – 80%.
In both cases the timber is untreated and either option would have the same life expectancy of around 200 years but there are many examples of oak framed buildings still standing for up to 500 years.