I think it takes as long as it takes. With a young big dog and a kitten, you are probably going to have to watch them for a good long while, because both of them are likely to want to play, and it's SO easy for the dog to be too rough or the kitten to catch the dog with a claw, even if they have the best intentions. It sounds like things are moving the right way from your description - but kittens are so skittish!
I have one cat who insists on playing with my young lurcher, and having had them both since last Feb, I eventually gave up trying to stop them because they both love it and are very careful. But it was I think at least a couple of months before I came to that conclusion, and I'd be more careful with a kitten because they are so fragile. My cat and lurcher are best buddies and leap about like loons.