I used to free-feed Levi and he would self-regulate to about 1/4 to 1/2 cup per day. I usually had to entice him to eat...and then my brother and his dog moved in with us.
Ever since, Levi has become a little piggy and we have switched to scheduled feedings.
I give him 1/8 cup of kibble twice a day. He gets a lot of (small) snacks and training treats, he sneaks the bigger dog's food, and he scrounges for crumbs constantly. He has a very lean body and maintains his weight well with this amount of food (he weighs 5.5 pounds and is super active). Sometimes I'll up one of his meals to 1/4 cup if we are particularly busy; sometimes I'll mix in about a tablespoon of wet food as a treat.
If I let him free-feed now...there is no telling how much he would try to eat! He'd probably eat until he got sick.