If your talking about a single female, then yes they spawn out all their eggs over a few hours. Once they start ovulating they continue until spent. As for the whole Walleye population, they usually mature over about a 2-3 week period. There is usually a a few days where there's a peak and a large part of the population spawn during that period. That's usually a 5-7 day period of peak spawning. It doesn't just happen at night. usually young fish show up first on the spawning grounds and are less successful then prime fish in optimum health.