How much lactose?

I have two 5 gallon batches of cider going right now. I am going to be kegging them in about 10 days. They are treetop apple juice, and one has 2 lbs of corn sugar added, and the other has 1lb of brown sugar and 1 lb of honey. They were fermented with Muntons dry yeast. I think I am going to bring at least one keg to thanksgiving, so I want them to be fairly sweet and good for the family. I was planning on using lactose for this. I know that I can add some to one glass and see how sweet it is when I transfer to the keg, but i was hoping someone can give me a ballpark idea of how much will be required for 5 gallons.

I'd start with 8 oz., but buy a pound. Since you're drinking it without waiting for it to age and with that high an ABV, you'll probably need the extra.

I have 2 pounds for the two 5 gallon batches. It might not have a ton of time to age, but it was started awhile ago. The total time from start to when I want to drink it will be about 5 weeks. I'm not sure if this changes anything.