Basically I'm looking at doing an extract brew with carapils. Due to issues with time I was going to cold steep the grains and then boil after 24hrs. I'm wondering how much carapils I should get for a 40 pint brew. Would 1kg be ok and what I need to add some dextrose?
I wouldn't recommend cold steeping, at the heat of the water is an important part of extracting all the sugary goodness that is inside the grain. Typically, you'll want to steep the grains in 152F water for at least an hour before you bring the wort to a boil. I have read some forum posts in the past, though, where the wort was left to cool overnight before boiling the next day. Just make sure you take the grains out of the water, or you may get some undesirable off-flavors!
As far as 1 kg being too much or too little, it really depends on the rest of your recipe and how much extract you will be using.