How is possible to know how much time Tiny OS takes for its own work?
Too much.

I know of no application that fit on a '51 that should have an OS.

Please understand, I have no problem with a RTOS if the project is big enough and run on an appropiate processor which never should be the case with a '51.

If you need an OS you need go to a different processor.

The architecture of the '51 makes applications geared towards the '51 run like dynamite, but it does not lend itself to an effective multitasker.

A lot of people try to wrestle a processor architecture they like into doing something is is not a good choice for, the fact that Keil has been "forced" to make a RTOS for the '51 clearly states how many misguided individuals have applied marketing pressure.

Erik