I use the Garmin Express tool that notifies me a map update awaiting. Then I hook up (USB), a few clicks of the mouse, check back in a few hours and it's usually done.
I don't know how it works... it just does.
If it wasn't for some of those tools, I probably wouldn't bother.
Have a similar app tool for radar detector database, but I receive weekly email notification for which states have updates.
So I figure the Garmin is doing all the states it takes a few hours / detector a few states take minutes instead of hours.
I guess internet speed and how large the update is determines how long it takes.