Its like a sliding scale where the ratio of flavor and aroma changes. So around 20 minutes would be mostly flavor and some aroma, whereas 5 minutes is mostly aroma and less flavor. I would think that flavor is added in almost any addition after about 45 minutes. You gotta remember though, our sense of smell is closely knit with our sense of taste. So IMO, you get flavor and aroma from any late additions...
I don't think there's a cut-off where you can say that this addition will not effect aroma or flavor. Even hops added at 60min will effect aroma, and hops added at 0min will effect bitterness. Really the only hops that will not effect bitterness are dry hop additions, but like soulive mentioned smell and taste may trick you into thinking its actually got more IBU's than it really does.
If you had to guess, pick flavor or aroma (if you think the split is between 65/35 and 50/50, say both). My goal is to adjust the majority of the aroma hops to FWH. I realize that there may be some overlap, but I'm just looking for a starting point.
FWH is at the other end of the spectrum from aroma adds. One member of OBC did extensive research into FWH and concluded that is made a slight difference in the bittering profile, but none for flavor or aroma. He was going to write an article for BYO about it, but decided there wasn't any point.
What we call flavor/aroma are a spectrum of oils. Heavy ones are flavor, light ones aroma. A flavor add at 20 minutes only means that the aroma oils are evaporated during a 20 minute boil. For an aroma add, the flavor oils are there, but dominated by the light, volatile aroma oils.