They vary from about 1/4' up to 2" depending on what type you use. Remember that with laminate, you can go the heavier pad that will give you a bit more lee-way as far as how perfectly flat the floor is below it.
Is your current floor wood or concrete? If wood, is it plywood or lauan? Is the un-evenness due to bad seams, or is it damaged wood?
It's doubtful that you'll use the pour in type; you'll probably want one you trowel in.
Here are some various floor levelers you can look at - and learn a little about as far as their recommended applications go:
http://www.google.com/search?num=100...ng&btnG=Search
...or check out the couple different ones they carry at Lowes/HomeDepot. All will tell on the packaging what the maximum thickness is.