My brother migrated to Australia when he was 10 from HK, he refused to say anything but single words at school for almost 1 year. After, when he started to open his mouth, he speaks perfect Aussie (not English). Everyone thought that he was born locally in Australia. He is surrounded by his Aussie mates. His partner is non Chinese. He is a real banana.
He has been living in Australia since (nearly 30 years up to date). His Cantonese is almost non existent, apart from single-word responses. He does not hear/speak/ read or write any Chinese at all in his daily life, except when he comes back to HK for holiday occasionally.