At what water temperature should you start chlorination in the spring?

The chlorine level in the pool should be maintained at 1-3 ppm throughout the year, and shocked about once a week with either chlorine or a non-chlorine shock. During the winter, the chlorine level will tend to stay about the same unless the pool is heated, but the chlorine level must be maintained, and the pool must be shocked to destroy the chloramines(ineffective, combined chlorine), and algae spores which are not visible in the water.

The previous answer is correct that the chlorine level should be maintained to be between 1 and 3 ppm. In my pool, I have found that summer usage of chlorine is around 0.5 ppm/day, but we keep our pool warm (solar heated) at around 88 degrees and higher temperatures will cause chlorine to evaporate more rapidly. We use a pool cover.

In the winter, the pool is not heated and the water typically drops to around 55 degrees (I live in northern California so it doesn't get too cold in the winter). The chlorine usage drops to less than 1 ppm/week (about 0.15/day). I doubt very much that you could close up your pool with a large amount of chlorine and have it last more than one month so you should be checking your pool at least once every 2 weeks and keeping the chlorine at 2-3 ppm for minimal maintenance.

I live in South Louisiana and I do close up my pool for the winter. In October, the pool is drained to a level just below the skimmer. All water is removed from the pipes to keep it from freezing. The pool is then winterized. The pool is thoroughly vacuumed. A dilute solution of copper (II) sulfate(algaecide) is added, the two floaters are filled with 3 inch chlorine tablets, and 4-5 pounds of lithium hypochlorite (shock) are added. The winter pool cover is pulled over and tightly strapped down. The pump is shut off. I only check on the pool cover occasionally to see that it is still tied down and I remove any excess rain water on it. In March, the winter cover is removed and the water is beautiful. Each time I have had it tested, it's ready to swim. We put the solar cover on it and in a few days, we're swimming. It's easy as can be!