I'm not certain as I haven't used dial-up in awhile, but I remember that anything over 1 meg or 2 just wasn't worth my time to wait for it. You can always use tools to lower the size/quality of the video specifically for dialup users (many websites have low bandwidth and high bandwidth versions of their vids) this way, those that want to wait, can, those that want to see if it's worth waiting for can download the smaller version first.
Hey, found a nice chart for ya. Assuming that dialup is approximately 56k or less, you're looking at 2+ minutes per megabyte.
http://pdacrsp.oregonstate.edu/pubs/acrotime.html