The repeating cycle of a repeating decimal for a fraction with a denominator of 7 can be up to 6 digits long. This is because the length of the repeating cycle of a decimal is determined by the smallest power of 10 that, when reduced modulo the denominator, gives a remainder of 1.
For the denominator 7, the possible lengths of the repeating cycle can be found by determining the order of 10 modulo 7, which is 6. Hence, the correct answer is:
up to six digits long.