1 oz per gallon will barely take the edge off dryness. You might not even notice it.
1.5-2 oz per gallon will definitely take the edge off.
2.5-3 oz per gallon will begin to give the cider some sweetness and body.
3.5 oz per gallon is the maximum I would recommend and will taste quite sweet.
More recently I've been recommending xylitol, which is another unfermentable sweetener. It tastes just like sugar and is sweeter than lactose. If using xylitol, try about 0.5 oz less per gallon than recommended above. Warning: Xylitol is extremely poisonous to dogs, so be careful with it. But tastes exactly like sugar, which is nice.
Cheers and good luck!