1. The regulations (or codes) in your locality or country
2. The soil resistivity at the turbine location.
In the UK, the main reference standard is BSEN 62305 (formerly BS6651) - "Protection Against Lightning" and requires that the local earth is 10 ohms or less. The resistance may be different in other countries.
The soil resistivity varies widely depending on moisture content and soil composition. There are 2 main ways to ensure you end up with an earth electrode of the target resistance or less - Firstly, you can measure the soil resistivity at the target depth, and calculate the amount of copper you need to achieve that final resistance, or, you can put some copper in the ground, measure the resistance of the copper to earth, and if too high, keep adding more copper till the total resistance comes down to target.
If you have too high a resistance, caused either my insufficient conductor in the ground, or too high soil resistivity, then if your turbine gets hit by lightning, the lightning may well travel down the cable to your house rather than dissipate safely into the ground.