The repeating cycle of a repeating decimal for a rational number is determined by the denominator when it is in its simplest form. For a denominator of 7, the cycle can be up to 6 digits long.
This is based on the property that the maximum length of the repeating cycle of a decimal representation of a rational number \( \frac{p}{q} \) occurs at \( q - 1 \) if \( q \) is prime and does not divide 10. Since 7 is a prime number, the repeating cycle can be up to 6 digits long.
So the correct answer is: up to six digits long.