Dogs sometimes have health problems related to abuse or neglect, or just from being in a shelter. Very few have much if any training or socialization. More often than not, there may be significant behavior issues to overcome, like barking, marking, separation anxiety, extreme shyness, or just a general lack of control. We feel very lucky when we get a well-cared for, partially trained, well adjusted dog with a good temperament, but we actually sometimes do. This takes time and effort.
Dogs are in foster homes for anywhere from two weeks to a month or two.
Very, very infrequently is a dog in rescue for less than two weeks, and dogs in foster care for longer than two months are rare. So you will generally need at least two weeks to be able to properly evaluate a dog. Furthermore, if the dog does not have a previous health record or it comes from a shelter, it takes at least two weeks to properly evaluate its health condition. Dogs that need a great deal of basic training and socializations take longer yet. And some dogs are simply harder to place than others for reasons we are not aware of and so require a longer period of stay in a foster home.