For me, I give him both sides for 15 mins each! I've tried just to feed him with 1 side but after 10-15 mins, he'll ask (make noise) for the other side!
If your baby doesn't ask / want another side, that means 1 side is enough!
since your milk supply has been established, no need to feed him both sides unless he is not satisfy.
you should feed on side only, regarding the time,
it is not fixed, because if your baby is still small, he may need at least 20 mins, however, if some babies over 3 months, they can only take 10 mins .
anyway, if your breast is very soft and empty after feeding one side, you can offer another side to him if he still need.
both foremilk and hindmilk have enough nutriation for baby, foremilk contains more water , while hindmilk has more fat, no need to squeeze out the foremilk, it is the wrong instructions by the old people.