About 12 inchs from the turbo will be fine.

Make sure the wires are pointing upwards (and not less than 25 degrees to horizontal) to ensure any condensation drains out of the sensor.

Heated sensors will reach operating temperature in approx 20 seconds so it doesn't really matter if you can't get very close to the turbo, as long as the air passing the sensor is only from the engine and no air is drawn in from exhaust joints etc.