This google Chrome Extention will add an indicator to your Youtube Video Player, that show when the Video will finish.
The time will adjust depending on your playback speed.
Sadly I am not able right now to create an google developer account, so I cannot publish it in the google web store.
To install the Extention clone the repository. Then enter your browser Extention-Menu und enable Developer mode. Finaly import an unpacked Extention.