My newest boy took about 3 weeks. He has been with me now since February and still cries often. He was home for a week sick and when he came back it was a huge adjustment all over again. Today he cried for a bit when mom left. He hasn't done that for awhile. He then cried for the whole time we were outside. That drives me crazy! LOL He then cried off and on during the day. He slept the full nap time thank goodness!
I did terminate one boy once after 3 days. I just couldn't stand his scream, he wouldn't eat, wouldn't sleep etc. So on day 3 I told mom it wasn't going to work.
Do what is best for you and your whole group.