Depends on what type of lungfish you have. I have an African lungfish, which is the largest of them all and his max length can be up to 6 and a half feet. Mine is about 3 feet now and 6 yrs old. I have read of some lungfish reaching well over 20 years old. I'm not sure how long the average age is tho.
6yrs old 3ft. thats a WAL, the MAL -Protopterus Aethiopicus Aeathiopicus get 6.5ft., the Protopterus Aethiopicus Congicus get about 4.5ft., Protopterus Aethiopicus M.(?) get over 6.5ft maybe close to 7ft. I believe
the west african lungfish is the longest, 7 feet. one of the austrailians is the heaviest like 80+ pounds, the one mentioned above. the africans need 1000+ tanks in my opinion.