The repeating cycle of a repeating decimal formed by a rational number with a denominator of 7 can have a maximum length of 6 digits. This is because the length of the repeating section of a decimal is determined by the properties of the denominator when expressed in its simplest form relative to 10.
In this case, since 7 is a prime number that does not divide 10, the repeating decimal can have a maximum length of \( 7 - 1 = 6 \) digits.
Therefore, the correct answer is: up to six digits long.