When does a passenger plane leave the cruising phase? I have Flightradar24 data here. Which are in the following structure.

latitude    longitude   altitude    lastUpdate      velocity
....
49.34240    8.77556 19650   2018-11-01 09:38:09     415
49.35040    8.77490 19550   2018-11-01 09:38:15     414
49.35617    8.77442 19475   2018-11-01 09:38:20     414
49.37045    8.77308 19275   2018-11-01 09:38:24     412
49.38132    8.77208 19125   2018-11-01 09:38:30     411
49.38964    8.77132 19025   2018-11-01 09:38:34     410
49.39796    8.77054 18900   2018-11-01 09:38:39     409
....


This is data for a specific flight. My goal is to split the flight in two parts.


Pt 1: Airborne -> Exit cruise time / start Approach 
Pt 2: Approach Start --> Touchdown


Well, I am not to deep into aviation. My naive approach would be:


  If the flying altitude has dropped continuously over the last k
  points, then the first timestamp is the approach start.


Now I ask myself, in what period of time does a passenger plane leave the cruising phase?

This is different from the pilot calculating the top of descent explained here: What is a rule of thumb for when to begin a descent?

How much ft altitude does an airplane usually lose when it comes out of the cruising phase? And how long does he need for that?

The background is that I want to understand how long a passenger plane usually takes to land at a specific airport from the approach phase. Optimally, I then see that on days when the airspace over the airport is fuller on other days, times, the approach phase takes much longer. Or do you think there are other factors playing an important role? Or is the approach not so correct. Maybe it would be better if I do not look from Approachphase, but from the time where a certain radius was reached. 80km before arrival?
The distinction when you cross from one phase to another is somewhat arbitrary.  However, you can slice this multiple ways depending on your goal:  The approach phase could be considered to start at the first point of a published STAR, (Standard Arrival) the initial approach fix of a published instrument approach procedure, when the pilot descends below the class A airspace, i.e. &lt;18,000' MSL, or the first ATC assigned vector off the filed route.  (Or some radius as you suggested...)  

If you really want to dig into this my recommendation is to go to a local pilot shop and purchase a set of Approach Plates/STARS and a high altitude IFR chart for the area you are interested in.  You will likely need help interpreting them if you are unfamiliar with these publications.  

If you then have a specific question on your airport of interest there are plenty of experienced people here who are willing to help, but a full education on high altitude IFR enroute and terminal arrival procedures is beyond the scope of this forum.  

In lieu of that, if you don't want to take that kind of time to learn something foreign but have a specific airport you are interested in analyzing, please let me know.  I have a full set of United States procedures loaded in my iPad EFB and would be happy to post a screenshot with an interpretive description of where I might consider the approach phase to begin for a specific airfield/runway combination.  

There are many many different arrival procedures nation wide, into airports of varying levels of traffic congestion.  Without a better understanding of your actual goal the question as stated is almost to broad to answer, especially for someone who is admittedly "not too deep into aviation".  Good luck with your project.