This will be influenced by a few factors.
temperatures -- higher = change it more often.
Light exposture -- more = change it more often
How quickly you have to top-off and how long it sits at lower volumes etc... The greater proportion of 'fresh' added water+ferts the longer it'll take to get nasty.
I don't have a recycling system but i do have a 55gallon rez. I noticed long ago not to leave it at lower volumes for any extended period of time. Exacerbated by warmer temps as the powerhead used to agitate/oxygenate will warm a smaller volume a bit more than normal but also because the microbes/liter amps up that much faster with greater surface to volume ratio.
I use mine in bunches, yours is depleted in a more spread out way, even if the same amount drunk per increment of time on average. So i'd top off at 50-75%.
If you want to learn how far you can push it, or compare procedures to see which last longer between necessary full-refill/cleaning, Just go until you notice any sort of biofilm or hint of a new smell. It's usually pretty obvious when water starts to turn - this won't derail a grow. Now, you know relative to your SOP and environment how long you can go. this helps for planning - like when taking a vacation you'll know if you need to do an early cleaning to avoid issues by the time you get back.