I can usually go 1 week without stressing. The only limitation I have is usually my ATO container (a 5 gallon jug). Longest I have gone is 2 weeks, but I had someone come by and switch my ATO containers.
I am getting ready to leave my tank for 5 weeks. Yup, you heard that right... 5 whole weeks away. I tested out the evaporation and believe 20 gallons of top off should hold me for that time. I am also using an eheim feeder, webcam, portal, and android app to keep an eye on my tank... but you know