If you already have the logo stored (ie. not creating it from scratch and don't have to clean it up)... maybe 5-15 minutes depending on how in-depth you want it (fading in/out, colors, etc.). The more "flashy" you want it, the longer it will take.<BR>This assumes you already know Flash.
We have a dedicated multimedia girl here and it takes her 1 day per scene if you want it to look good with music and effects. I just took our Intro. Flash class she teaches here and it would take me, a novice, probably 1-2 hours to do something basic like your talking about to create something long and good enough with effects. :)