Its easy to calculate....
Imperial Star destroyer and Falcon move about same speed in normal flight (ESB chasing scene).
Then falcon turns back and moves to "attack position"
Falcon--> <--Destroyer
It took a stopwatch and Falcon travelled about 5.7 seconds from its position to Stardestroyers head. This alone doesnt tell much, but as we remember a few scenes back, there were 2 Destroyers pursuing Falcon. Now, as Falcon outmaneuvered them, they were forced to "take evasive action" and nearly collided at chaising speed. (Same as Falcons)!
I took my stopwatch again and it took: 11.2 seconds for both ships to be "on top of eachother".
So they both travelled at the same speed to the opposite directions.
So its safe to say that With only 1 ship moving, it would take 22,4 seconds to move its own lenght.
Now, lets check from Starwars.com databank how long is a star destroyer....
http://www.starwars.com/databank/starship/imperialstardestroyer/
It seems to be 1600 meters long. Okay, so Imperial stardestroyer travels 1600 meters in 22.4 seconds.
S = T*V
V = S/T
So.
V = 1600 meters / 22.4 seconds.
V = 75.43 m/s
Okay. Now this gets curious. From Wikipedia I saw that Bespin is located 49,100 light years from the galaxy core. This alone wont help, but as they were pretty near Hoth when they entered the asteroid field (no hyperdrive, remember?) Ill take their position roughly. So Hoth is located about 50,250 light years from the core.
So there are 1150 lightyears between them! And as they were pursuing the falcon, Im sure that both Imperials and Han gave their best speed at sublight.
So. 1150 lightyears is about: 1.08796077 × 10^19 meters by google.
So 1.08796077 × 10^19 meters / 73,43 meters / second
= 148162981070407190,521585183167643 seconds divided with 60
is 2469383017840119,84202641971946 minutes divided with 60 is 41156383630668,664033773661991 hours divided with 24 is
1714849317944,52766807390258295833 days divided with 30 is 57161643931,4842556024634194319333 months divided with 12 is
4763470327,62368796687195161932778 years. divided with 1000 is
4763470,32762368796687195161932778 thousands of years divided with 1000 4763,47032762368796687195161932778 millions of years!
So about 4763 Million years so about 4,76 Billion years!
Yeah, And Yoda trained Jedi for 900 years? Sounds cool. But hey, these are the facts.