I keep mine in a dark medicine bottle in my tank stand where it gets limited ambient light. As I use it near-daily, I want to keep it close.
Oh, and I mixed mine 2 tbsp per 250 ml so I didn't have to dose as much. So, my batch will need to last longer. I guess if the plants stop reacting positively to the daily dosing, I'll know it's time for a new batch.
46g bf, XP3 w/ vertical spray bar, CO2 w/ SMS122 & diffuser,100% Flourite, 96w+55w cf AHS, AGA Member