Now for the bad news. You need sunny weather, or two days of overcast sky, to reach the maximum effectiveness. SODIS becomes less effective as you travel away from the equator. You cannot use it in rain; it offers no residual disinfection; it may be less effective against bacterial spores and cyst stages of some parasites (similar to chlorine); and the water and the bottle need to be clear. If that wasn't bad enough, this method does nothing to help with chemical contamination, and only small bottles can be processed (the bottle must be 2 liters or smaller). If you have a clear glass bottle, it should last for years. But if the bottle is scratched up or made from milky plastic, less sunlight gets through, making this method more questionable.