Found this on another website asking about stainless steel hip flasks... keep in mind rum isn't 100 proof its about 40-45 proof which bacteria would still find it hard pressed to survive. Also I'm not sure about pre-opened rum but most alcohols get better with age.
Limp moose wrote:

man every thing these guys above me are saying is ridiculous. I have used hip flasks for years.
First off You can keep alcohol in it as long as you need to. It's stainless steal. And its closed. Its not going to suddenly corrode and turn into rust. What do you think aircraft drop tanks are made out of (hint its steal) Now depending on what you put in there it might go bad if you leave it out in the sun or something but I have kept whiskey in my flasks for over a month and it is still good.
when you do empty it you will want to clean it. Hot soap and water and a bunch of pipe cleaners can do a good job. Lets not forget you are storing 100 proof booze in this thing. Any bacteria is going to find that a pretty rough place to survive. Just be sure to rinse it thoroughly
I also keep the flask under the drivers seat of my car. This may violate open container laws in some areas. But it has never been an issue. (i dont drink and drive and I can count on no hand the number of times my car has been searched. ymmv)
The best time to use the flask is when you are going out and don't feel like spending a lot of money. Load it up with whiskey or rum and just order cokes then add your own booze and blamo you can drink rum and coke all night and only pay 3 bucks.
So that flask is pretty cool learn to enjoy it.
One last thing they work best if you carry them in a jacket breast pocket. If you keep it in a back pocket like a wallet they get to warm from your ass heat and they are uncomfortable to sit on.
Also the TSA won't let you take a full one on a plane (ass holes)