@DaNi
what is the Quake command for enable the FPS counter?
There is no FPS counter to enable, per se.
You can, however, run demo map with *average* framerate counting enabled (total number of frames rendered / time in seconds).
To do so use TIMEDEMO command. It must be given the map name as argument. There are 3 demo maps to time. Traditionally demo2 is considered the standard for comparing framerates.
Type this in console to calculate the average framerate of demo2:
timedemo demo2
Timerefresh is useless for benchmarks as the result greatly varies depending on the position of the player and the direction the player looks at.