http://i307.photobucket.com/albums/nn298/newhope6754/skyrimmap-Copy.jpg
According to the Quakecon demo (used because it is uncut & played live) It takes him about 18 minutes to get from point A to point B (subtracting the time spent in the dungeon). Todd was taking his time however so i figure it will be about 10 to 15 minutes if you really pushed.
The approximate path from Falkreath to Solitude can be seen here. Ive taken approximate guesses of the same distance and added them up to see how long it will take to get from Falkreath to Solitude.
http://i307.photobucket.com/albums/nn298/newhope6754/skyrimmap-Copy-Copy.jpg
So it will take about approximately 40-60 minutes to get from Falkreath to Solitude. It will be about double that to go from
Riften to Solitude which equals about a two hour journey across the map. This is without stopping to fight monsters, talk to NPC's...ect