I like to leave a few sprigs of mint leaf in bottled water overnight to give it some extra flavor. It's delicious, but recently I left one in the fridge while I was on vacation for five days. I tasted the water and it still tastes great, but I'm curious how long something like mint needs to soak in (refrigerated) water before it becomes a safety concern?
I recently asked this question to my dietitian friend at work, and she advised fruit/herbs last about 3 to 5 days in water in the fridge before becoming soft and mushy. If your mint leaf still looks nice and pretty, you are most likely OK. She also recommended a website to me: www.infusedwaters.com/faq for recipes and advise.
I keep a bunch of sprigs in the water jug at my desk and discovered that if kept near sunlight they actually will continue to live and even grow to produce new mint. The existing sprigs have grown a network of roots. When a leaf gets bruised or falls off I take it out before it starts to rott, and have been drinking with the same mint for a month now. Once a week I give them a little rinse with dish soap to make sure nothing evil grows on the stalks. Make sure you rinse well before putting them back in water to drink, unless you like the taste of dish soap. ;) Enjoy!
Disclaimer: This is my own experiment. I am not a doctor, a scientist, etc. Take all advice with a pinch of salt, and a sprig of fresh clipped mint.