You shouldn't need to toss the oil after frying a single batch of french fries. Strained, cooled, and kept in a sealed container, the oil is usually good for a couple more deep-fry sessions, boogiebaby says on Chowhound. Naturally there are other factors to consider, as Chemicalkinetics points out.
The higher you heat the oil, the quicker it'll degrade. And a highly refined oil like Wesson stands up to reuse better than a less refined one like cold-pressed canola oil. Of course, if you deep-fry only once or twice a year, say, your oil might turn rancid before you need it again. Give it a good sniff before you proceed.
Discuss: Reusing Oil Once?