Variations in Reference Material*
*Because there is very little information, I have also included statistics on Cape Parrots since so much of the research was done before these birds were reclassified into different species. This should still give you an accurate estimation of size and lifespan. There have also been huge jumps in quality of parrot care in the last decade, making lifespan drastically increase with proper care.
The average weight I could find was 313 grams, but my companion bird hovers exactly around 250 grams... I guess she's just really small for her species even though shes very fit.
So, I would say the average findable lifespan for a Brown-necked Parrot in captivity is about 33 years, but these aren't the most credible sources with some estimations seeming alarming low. My avian vet said that with more research in care standards I should expect 35-40 years.