How can I know in advance how much time will pass during Fast Travel?

Save the game, press 'T' to check the day and time, fast travel to the place, press 'T' again to see how many days have passed. That will tell you how long the fast travel time is between those two spots. Re-load the save if you aren't happy with your purchase.

My guess is the game uses a distance between two points on the map. Though they do have data on the main roads for travelling NPCs so it's not impossible that they also use that for more "realistic" estimation of the travel duration.

I know at least the timescale is involved. I sometimes use set timescale to 3 when in town to be able to trade and craft and leave town early afternoon instead of late at night. If I forget to set it back to 20 before fast travelling, the travels take much less time.

I suppose you could measure how much time passes for small, medium and long distance travels on the map and use that as a rough estimate for other trips. Otherwise it's hard to describe. For example I know it takes 2-3 hours between Riverwood and Whiterun. But I couldn't tell you how many in-game kilometers there are or how many pixels separate both locations in the game map.