funnily enough (although i am not laughing...) I just had a small cold/flu. I woke with a sore throat and runny nose last saturday. By the following thursday, the cold symptoms were pretty much gone. On the thursday (but more definitely on the friday, ie 6 days from first flu symptoms) I felt like i was having a relapse. I took a single shot of 75mg prednisone, and it seems to have stoped it in its tracks. And from memory, i usually would have a cold for a week, be on antibiotics for that week, then hit it with 1 week of prednisone to minimise the ensuing relapse. So 6-7days seems about normal for me.
As for only existing symproms appearing after a cold/flu, i would be pretty sure that new symproms have appeared for me after some cold / flu's. The reasoning being that I can only remembver having flare ups after a cold/flu, and so this would be the time a new symptom would appear for the first time. Its hard to say what is a new symptom, when i have most symptoms of ms to some (minor) extent. But, the previous major relapse definitely incured new symptoms, and occured during the cold/flu which lasted more than a week.