How long is the lifespan of an automatic transmission?

It is very difficult to answer this question. Car manufacturers design (taking into account the average user) transmission as weak as possible and as durable as necessary to ensure minimum costs during the warranty period. They are trying to operate opposing parameters.

But in general, the lifetime of the transmission is not so much about the value of the mileage, but about how the mileage is achieved, perhaps more importantly how and for how long it is loaded:

That puts more pressure on the automatic transmission
Higher ambient temperature
More gearchanges (city traffic)
More gears on the transmission, e.g. 6,7, 8 and 9 gears
Difficult conditions (transporting a trailer, mountains, etc.)
Start-stop mode (more gear changes and higher temperature)
A sporty driving style
A more powerful engine

Also, the type of the transmission (CVT, DSG, planetary – look here) determines how much resources it has. As well as on what vehicle it has been installed.

The easiest parameter to measure would be the number of gear change, the load and the temperature.

It is clear that no one reads them and therefore it cannot be used. Therefore, generalized mileage and kilometers are used, taking into account the average user.

Because transmission faults are divided into three major groups, it is very difficult, if not impossible, to say how long is the transmissions life span. At the same time, there is a transmission that has been driven for a long time without any oil change and the problems have not yet materialized.

A distinction must be made between wear of the transmission (this is a fact when the oil is not changed) and the symptoms of wear. However, when it comes to the most common wear-and-tear errors (which can be avoided with maintenance), it can be pointed out that most of the symptoms (depending on driving style and load) begin to appear.