A very well built castle will last "indefinitely".
Older "castles" may last longer than more recent ones.
The Romans learned to build in real concrete maybe around 100 BC.
The Colosseum is concrete - it has/had brick veneer.
The last castle to fall in the English civil war was Castle Raglan in Wales in 1646.
The main walls were 14 feet thick in stone. The free standing 'castle in it's own right' Great Tower was "slighted" subsequently but proved to hard to destroy and was left as was.
The tower wall breach was a product of labour intensive "slighting". The tower's top storey was removed but, when this proved excessively labour intensive, the tower was undermined with a propped tunnel, and once the foundations were gone on one side fires were lit to destroy the props - see photo below. Without this process I'd expect those walls to be good for far beyond the 400 years that they have stood so far. Based on the Colosseum ('pure' concrete) a lifetime of 2000+ years would not be unexpected.
The tower, and the main castle are still standing today - VERY solid - if it had not been 'slighted' at the time it would be an INSTANTLY viable castle now. You may need to add a new drawbridge - it is accessed on the 2nd or 3rd level from the main castle over a moat via a drawbridge. If that has fallen in over the years it makes it almost instantly defensible.
I have many photos of Castle Raglan (taken in 2003).
A few only
Access to the Great tower:
Cross sectional view.
Floors would have been destroyed at the time.
You may wish to posit stone floors of some sort to allow survival.
Spiral stairways are stone slabs set into walls. Some survive, some have collapsed - try not to be on a collapsing one.
A less well built castle - lifetime - not so long.
Macduff castle - my photos from 2003 in Scotland (needless to say).
Location and more photos