three years
It may never fruit and if it did it would be thousands to one that it would be any good.
like all tree crops mango isn't normally growen from seed, it is either rooted or grafted. the reason being two fold: grown from seed the plant has to go through a period of growth called juvenillity...
it takes 6-10 years for it to grow edible lemons
Approximately fifteen years, compared to five years for a grafted tree.