The simple answer to your question is - change your reservoir once a week.

Some say two weeks and some even three. To avoid nutrition imbalances, reduced yields etc. you should change your reservoir once a week. The size of reservoir will also determine the frequency of reservoir changes. If you have greatly oversized your reservoir then 2 weeks will be ok, but if the reservoir is too small then you will need to change the reservoir more frequently. Type of crop will also influence eg., high feeding crops (tomato/peppers) vs low feeders (lettuce, other greens).

Last summer I grew lettuce in an NFT system (25 plant sites) and I used a 5 gallon bucket as a reservoir. It was to small and needed to be changed every two days. I then changed to a 25 gallon reservoir and had much better results, changing the reservoir at once a week.

Also keep in mind that the plant feeds every day and as nutrients are consumed (some at higher rates than others) the levels of some of the elements drop and thus the plant begins to starve. This happens even if the EC/PPM seems to stay the same and sometimes it may even increase (plants also consume water which can cause higher EC/PPM).

Another problem is that when nutrients are in solution for a "long/longer" period of time some will react with each other causing a precipitate. Calcium, sulfur and phosphates are the main culprits, especially when the pH of the solution begins to fluctuate. Other minerals will oxidize and some will volatilize. All of which causes nutritional dis-orders.

In summary, change your reservoir once a week, but keep an eye on EC/PPM and pH. Keep the values within range. Good luck.