I've known as late as a year, but in any case, there is no rush. Yes there is a slight increased risk of testicular cancer with retained testicles due the the body heat. But it's not a big risk, and in any case, it's not really something that is going to happen over night. If you do need, or decide, to go down the route of neutering then these days the recommendation is not before long bone closure, which with Labradors is around 14 months. Add a couple or so months to that to be on the safe side and Tarimoor's 18 months is a good guide.