By period of time do you mean days, weeks or months? With a great bait tank set up, to include added salt, oxygen and the right temperatures, you may keep bait lively for 24-48 hours. Anything past that you would need a tank with a true filtration system such as those used for aquariums to address the nitrification process. Even then I would think they would still die from stress. There is a striper guide on Lake Texoma named James Allen. Nickname is shad cologne if that tells you anything. I fished with him several times and he always has the best and liveliest shad. He posted a video on youtube of how to keep shad alive and in good shape. Here is the link. How to keep bait alive video one of two how I do it series - YouTube. A really good 30 gallon shad bait tank will run $500-$600.