What is the acceptable value of earth resistance for the earthing system of a house or small building?

Max 25 ohms according to the NEC (US). Less is better. A good system can approach one ohm

The earthing value is small enough to let the earth fault to trip. For example a 63A MCB need 315A E/F to trip in 0.1s to 5s. For 220V, get about 0.7 ohm (Earthing + cpc + ...).

I'm not current on the NEC but is used to be that if the installer put in ONE grounding rod and the resistance to ground was greater than 25 ohms, he was required to put in a second ground rod.

If a second rod is in place, there was no "resistance" requirement.

"Smart" electricians quickly figured out that it was easier/cheaper/etc. to just plan on installing two rods in the first place and just not care about the resistance value.