The repeating cycle of the decimal representation of a rational number is determined by the denominator when it is expressed in its simplest form. For the denominator 7, the length of the repeating cycle can be at most \( 6 \) digits long.
This is because in general, the maximum length of the repeating cycle of a decimal representation of a fraction \( \frac{a}{b} \) (in lowest terms) is given by the order of \( 10 \) modulo \( b \). Since \( 7 \) is a prime number, the maximum possible length of the repeating decimal for \( \frac{a}{7} \) is \( 6 \) (which corresponds to the factors of \( 10^n - 1 = 999999 \) and \( 7 \) being \( 6 \) because \( 7 \) divides \( 10^6 - 1 \) but does not divide \( 10^k - 1 \) for any \( k < 6 \)).
Therefore, the answer is that the repeating cycle can be up to six digits long.