Short Answer: It varies based on the site, but try to get as low as possible. Try for <5,000 ohms for typical top soils. Ideally, target <1,800 ohms. Note that you can achieve excellent results with higher than 5,000 ohm contact resistance.
Long Answer: "Good" contact resistance may vary based on your survey location so there isn't a standard number of ohms to look for. Always try to get the lowest contact resistance possible to ensure clean data. We generally recommend trying to get less than 5,000 ohms for contact resistance—though we often recommend trying to get less than 1,800 ohms if possible. We have also written an article on how to lower contact resistance.
Tip: If you're using a SuperSting™ Wifi, it can check the contact resistance for each electrode before you start your survey.
With the SuperSting™, less than 1,800 ohm allows the full 200 watts of power to be used which is called Boost Mode. If all of your contacts are above 1800 ohm, the boost AGI generator power supply or boost battery will not actually be used. To maximize battery life, you may turn off the SuperSting™ halfway through the day and swap the main and boost batteries since the boost is typically not used all the time on any one line due to variable contact resistance.
The SuperSting will use the contact resistance during auto-ranging before each measurement to correctly set the highest current for optimal signal levels. Lowering the contact resistance simply allows more current which raises your signal higher over any noise present (higher S/N). Higher voltage will be ranged during high contact resistance. This will be more of an issue if your connectors are loose, not clean or the cables are third party built and lack proper isolation. Lowering the contact resistance allows more current to be injected as per Ohms law which in turn makes the data set more tolerant of noisy conditions from connectors and the site in general.
For Resistivity on Land:
As stated above, the ideal is less than 1,800 ohms or as low as you can get. However, try for <5,000 ohms.
For Resistivity and IP:
Aim for <1,800 ohms or as low as you can.
For Resistivity on Water:
Water is so conductive that you should have close to 0 ohms resistance.
Extreme High Contact Sites:
Excellent data has been collected with what would be considered as very high contact resistance. Some sites could not lower below the 30,000 ohm range as example after the operators struggled to lower the contact resistance from 80,000 ohms. Occasional HVOVL errors were reported at these sites. These sites had frozen ground, dry crushed rock and other similar issues. Foil pits, salt water and higher surface area from 2 or more stakes/pins at each position helped. More details here: How do I lower my contact resistance
Quick Tip: Improving Contact Resistance (with video)