This is a very CPU & I/O intense operation. It performs a brute-force testing of a lot of different forecast settings in order to determine the optimal one for each series. The time for this operation to complete determined by the number of series and the number of combinations tested, and is normally set up to be 6-18 hours.
This should not be performed on a machine doing anything else while the autotuning is running. The session running the autotuning must not be logged out or otherwise interrupted.
A recommended method of setting up the autotuning is on a dedicated machine and then copy the data directory to it before starting the autotuning.