The maximum length of a QSFP28 (Quad Small Form-Factor Pluggable 28) Direct Attach Copper (DAC) cable depends on several factors, including the cable design, wire gauge, and the specific requirements of the networking equipment. QSFP28 DAC cables are used for high-speed connections in data centers and networking environments.

In general, the lengths of QSFP28 DAC cables can vary, but they typically come in various lengths ranging from 1 meter to 5 meters or longer. The most common lengths are 1m, 3m, and 5m. The length of the cable is influenced by the electrical characteristics of the cable, signal integrity requirements, and the supported data rates.

It's important to note that as the cable length increases, the signal quality may degrade, and the maximum supported data rate may decrease. The exact specifications for maximum length and supported data rates are often provided by the cable manufacturer and should be checked against the requirements of the networking equipment being used.

If you have specific QSFP28 DAC cables or equipment in mind, you should refer to the datasheets and documentation provided by the manufacturers for accurate and detailed information on cable lengths and compatibility. Additionally, the networking equipment you are using, such as switches and routers, may have guidelines on the supported cable lengths for optimal performance.