It is safe practice to not share water between plants but if like me and other growers you do use this practice then we have used prepared water for up to a month with no ill effects.
Water generally becomes smelly if it is no longer fresh. Most people don't recommend sharing water. Full stop. But I know enough people who have never had issues do it for me to not waste my time unnecessarily.
So again if you want to be as good a grower as most think they are on here then never share water and make a fresh batch every 5 minutes for every plant - if you are doing it for the fun of growing then just reuse water for up to a month. Saves hassle but realize some people believe this to be highly risky.