I fostered Ty starting the beginning of February and adopted him in mid-March. I generally tend to have my foster dogs for a minimum of 6 weeks before adopting them out (besides myself). The reason is the dog I get as a foster often isn't the same dog by the time they go up for adoption.
I tell people to think about the Rule of Three:
First Three Days--shock, assessing, etc by the dog.
First Three Weeks--the honeymoon--maybe the honeymoon from heaven or from hell.
By the three month mark, you are probably seeing the real dog--or beginning to.
When I fostered Ty, he was a dream dog for the first 3 weeks. Lovely, cat and dog friendly, house trained. At about week four and five we saw some barking when I left the house. Not always, but it happened. I ended up adopting him because I just adored him and as he stayed, we saw more and more distress at humans leaving. And it became full blown isolation distress. Barking for long periods of time, panic, excreting in his distress, chewing and digging.
We had wondered why all his teeth were flat and now I think it was due to his anxiety--when stressed he chewed and chewed hard. We started him on meds and instituted measures to work on his anxiety when no human is home. We made progress. It was slow and labour intensive, but it was progress.
And had I adopted him out to someone at the five week mark, I would have told them that he had some off and on again anxiety about people leaving, but it hadn't been consistent. And he would have escalated in his new home and likely would have been returned to me. And maybe the rescue would have been accused of "hiding" things about him.
Well, no. Not really. It is one of the reasons I do hang onto my fosters a fairly long time. I DO know that the dog can change significantly as they settle. Some don't, but some do and some of the behaviour issues of long standing don't necessarily show up in their full flower until the dog HAS been with you for three months or more.
I'm glad I kept Ty. He was still a dream dog in many respects. I am glad that I did mention to the one potential adopter that he did have occasional barking festivals when I left and that I couldn't guarantee that wouldn't continue or escalate. And that he did seem to have the odd accident when I left--again on and off. I didn't tell them he had isolation distress because it hadn't shown up as full blown isolation distress. And then it did, and we dealt with it.
I say this because there has been some tendency to blame rescues for keeping behaviour a secret from adopters. And that may happen. But sometimes the behaviour hasn't shown up or hasn't shown up in a way that is worrisome or cause for more than a note that the dog has on occasion barked when left alone. Or the foster parent hasn't had sufficient experience to recognize the behaviour for what it is.
I know that some rescues don't hang onto dogs for long periods of time unless there is a known medical or behavioural issue from the get-go. I discourage adoption before the dog has been in foster care for at least 4 to 6 weeks so that the honeymoon is over and maybe the real dog is showing up--but it's not always my call. And since I usually DO foster dogs with issues--generally medical, but sometimes behavioural, my foster dogs often are with me for longer periods of time. But I think the case can and should be made for hanging in there with foster dogs for longer than a couple of weeks. It can be the difference between an adoption breakdown and accusation of secrecy and a successful adoption because ALL the parties knew the dog better.
Just my two cents from my side of the rescue world.