Since light and air are the 2 biggest enemies of nic base, it's a good practice to break it down into sizes that will only be out of the freezer for 3 months or so. For me that is 120mls but I also mix for 3-4 people. You'll need to do the math and figure it out for what you mix.
After 3+ months, the nic that is removed from the freezer will turn a little yellow. Totally normal and no big deal whatsoever. Just part of the oxidation process. I actually like mixing my unflavored with the oxidized nic. Has a nice flavor to it all by itself.
The out of the freezer time period for your working NIC can be extended to 5 - 6 months if stored in the refridgerator after each use. I in fact refreeze my working bottle during longer periods of non use and have not noticed any adverse effects.