I often use my own judgment since I am not a real pilot when to start descending. I have often wondered when do real world pilots decide to start descending for landing?

If this is normal descent (not a high speed) I use rule of thumb: Top of Descent is 3x height difference. For example, flying FL200 (20.000 feet above sea level) and descending to FL50 (5000ft ASL), I start my descent 45 miles from the runway. 15 x 3 = 45. This is the easiest way and usual enough. When flying STAR, usually they have minimums, sometimes maximums. I use 3x height rule also.

Depends what you are flying and what your cruise alt is, as well as the terrain you are flying over and the weather you are flying through. A very common rule of thumb for airliners is known as the '3 in 1 rule', which goes like this: The average airliner will lose 1000 feet for every 3nm travelled over the ground, so depending on how high up you are, you will know how far away from your destination you will be when a descent should be commenced. This means airliners can easily be 90 or more miles away from their destination when they start down, depending on the type and the terrain clearances.The 3 in 1 rule works for most airliners, but there are lots of other rules of thumb such as that for other craft, for example, on gliders, there is a 4 in 1 descent rule for landing out (i.e having to land in a field when there is no lift on a cross country flight). That rule goes like this: If you have to fly an impromptu circuit down to a field, you always keep your field of choice within a 4:1 glide ratio distance on the circuit, which equates to looking down at the field from your circuit at no less than 30 degrees. Most gliders can make well over a 35:1 glide ratio (even an airliner can manage about 7:1), but keeping the field at 4:1 means you will definitely make it in there regardless of wind if you use the spoilers, and there has to be a big margin for error, since a glider cannot open the throttle and go around, once you start for a landing, you have to continue it and get it right first time. I've used that 4:1 rule a number of times to get into tight fields. You need lots of spoiler for it to work well and you have to fly at about twice the stall speed and make very steep turns to pull it off properly.That 4:1 rule is broadly similar to the technique recommended for a dead stick outlanding in a GA aircraft incidentally.Al