Rather than a flat "three to six months" rule, size your reserve to your actual worst realistic gap, your slowest season, or the longest stretch a big client has ever paid late. A business with steady retainer clients needs a smaller buffer than one with large, irregular project payments, even at the same revenue level.
Want the full method? See working capital basics for the formula and a worked example, or check your Cash Conversion Cycle to see how long your cash is actually tied up.