A fostering relationship is just that - a relationship. No matter what the relationship is (romantic, family, friendly), there are always different factors that play into the whole "falling in love": How much time you spend with the person, how long you've known them, how well your personalities mesh, whether or not you have similar values and backgrounds, and how long you plan to know them. And just as all relationships develop at varying rates and to different degrees, the same goes for the relationship between foster parent and child.
I suppose this is kind of a trick question. There's no one answer, and there's no right answer. But as a foster parent of seven months, I can tell that - in my situation - seven months has been more than enough time to fall in love. But then again, by the time my birth children were seven months old I was also very much in love with them. No one ever seems to question that timeline though.