Which is the correct duration of an AES execution?
The duration will depend on the chip and on the implementation.

In a good implementation, the duration of encryption is independent of the combination of the secret key and the message to encrypt (otherwise a so-called timing attack to gain knowledge on the key and/or the message is possible).

You need a model of the chip and its cache and the assembly code to do some real estimates (usually giving you a range of minimal and maximal cycles needed). For tight bounds there are commercial offers, e.g., by Absint. It is a feasible, but not a trivial task.