From what ive read, the rate a walleye grows varies from lake to lake as well as where its located geologically. From what i gather there are several factors that are involved in growing walleye, or any fish for that matter. Forage type and availability, size of the water body, as well as the length of the growing season are some of the biggest factors. Cooler waters slow growth, but walleyes generally live longer and have a lower natural mortality as well as more abundant forage in these conditions because the cooler waters have less diversity throughout the predatory food chain in comparison to warmer water regions.
So, i guess it varies.