I want to bottle and carbonate in flip-top steel growlers. How long do you think the beer stays fresh? Any other problems that might arise?
I don't see any reason why you can't do this, just make sure you calculate priming sugar and make sure those growlers can handle the pressures you want to be at for your homebrew. You should use a priming sugar calculator like this.
The beer will stay fresh as long as you don't open it. For upwards of a year or more.