The length of the repeating cycle of a repeating decimal is determined by the denominator of the rational number when it is expressed in lowest terms. For a denominator of 7, the maximum length of the repeating part of the decimal can be at most \(7 - 1 = 6\) digits long, because the repeating cycle length of a fraction with a prime denominator \(p\) is at most \(p-1\).
Thus, the correct response is:
up to six digits long.