Hi,
From our experience here in the Philippines. For humid and rainy areas, it might take 6-8 years. For dry areas and with assisted watering on the plant, it might take 8-10 years even more. To make it fruit faster, add mychorrhiza, mulch and do double rootstocking using native langsat rootstock. Also, langsat also likes vermicast and vermitea. It might help in hastening juvenile stage of longkong plants.
Regards,
Berns