Distance does not matter, travel time does. As does pricing because prices for train and plane are basically unrelated and because I'm currently a student and don't have much money. My decision mostly depends on the events around my travel – sometimes I have to fly to make it in time. This means I find it difficult to give a simple function, so let me give some examples:

• I currently live in Edinburgh and I fly to visit people in Germany or the Netherlands over the weekend. On the other hand, I will move back to the continent in September and will visit the LW community weekend "on the way". This will involve a travel time of around 25h over a period of 45h (13.5h train, 6h ferry, 4h cycling, 1h waiting, ~0.5h local transport). If I took a plane, it would cost around the same and I'd spent 10h (1.5h flight, 2h security/waiting, 1.5h local transport, 5h packing and unpacking bike + getting the packaging in the first place). EDIT: Without the bike, the times would be 19h (train through the tunnel) vs 5h and I'd fly.

• There is a reasonable probability that I'll be living in Switzerland soon, in which case I'm planning to do family visits by ~9h train instead of ~5h flight+transport. That is because I'm expecting it to be possible to do "home office" in the train.

• I really like night trains. I count night travel as 2-3h time investment due to reduced sleep quality.

• I prefer flying + airport shuttle over driving a car for the same total time if I'm the only driver. If this is a road trip with like-minded people and we share driving responsibilities, up to like 24h of driving seem okay during holidays. I'd probably prefer the plane once I have a job.