There's no set-in-stone schedule where hydroponic water must be changed. In practical terms, changing hydroponic water can be more of a triggered event than a scheduled one. If you are encountering problems with a growing chemical imbalance, if there's been a buildup of debris from external sources that is clogging your system, if there are bacterial nuisances, or other (relatively uncommon) issues, these may trigger the need for a complete cleanout of your system and replacement of your hydroponic water.
That said, there are some situations in which scheduled flushing may be a practical choice. Medicinal crops are probably good examples of when preventative flushing is worthwhile. Some types of foods that tend to absorb certain minerals which can negatively affect their taste may be worth the trouble and expense of flushing. Small systems, too, can benefit from regular replacement, since problems in small systems tend to escalate rapidly.
