What map? Single country or region (like UK and Ireland) or whole continent (like Europe or the US and Canada)?
Also there are loads of other factors involved....
Your connection / Broadband speed? Wired or Wireless? Time of day? (TT's servers get very overloaded at times)
And finally, is this using the "Home" utility for older TomTom models, or the new "MyTomTom" program used by the Go1000/2000 and "Via" models.
For a download from "Home" of a single country, I would suggest roughly 15-30 minutes, and for the whole of Europe 1-3 hours (America is smaller) as ball-park figures,,,, but I've seen and heard of download times of 5-8 hours before now for the complete Western and Central Europe maps.