I know this sounds like a stupid question, but it hasn't been very hot here yet, but my system was down about 2 gallons (6 gallon open system), in about a week.
It's a very basic system, the plants are all doing very well (except the strawberries).
I added 2 gallons, with
nutrients for 1 gallon. I usually change the entire
nutrient/water solution every 3 - 4 weeks, so I still have two weeks to go.
There's a video on my blog:
Dan's Hydroponics - 2011
Dan