Not long at all, actually. The first time I added people, Google was recommending around 600 users. So I just selected all, the dropped them into a new circle. Same process to get the rest of the remainders over the next few days.

Now, Google shows maybe 100 or so recommended users, which is below my threshold of "fun number of people to add at one time". I say "maybe 100" because, interestingly, Google+ no longer shows the number of recommended users. I wonder if they removed this feature because of visual clutter, or were finding a lot of people abusing the feature like I did.