Nope, nothing really. The steel can impart a slight metallic taste over time and it will build up pressure when it gets warmer (which is why I never fill it all the way up to the top), but otherwise it seems like alcohol can be left in there for a long time without ill consequences. I would say at least a few months.

With that said, if you plan on using a traditional flask, you do have to work to find a good one. Most of the ones you find in stores today are of poor quality. Either poorly soldiered/welded, poorly sealing (of the cap), or both. The better ones are less likely to leak and break, however they're tough to find and usually pretty expensive.