Phytoplankton

[BlueAbyss]

If you let the culture go too long it will contain a lot of dead stuff and could potentially become contaminated with bad DOCs from the breakdown of said dead stuff. Not something you want to add in quantity to a tank full of expensive corals. That said, I'm sure a week is pretty safe, but its something to keep in mind.

[hillbillyreefer]

I leave mine brew until I can't see through the 2 litre bottle 2-3 weeks. Then shut off the air and let it sit in a cool place and settle for a couple days. Pour off the thin stuff on top and use it to restart the next batch and put the concentrated stuff in the fridge for the tank. If you really care florida aquafarms sells some sort of meter to give you a density, I think it is a colorometer(or something like that). They are around $10 bucks so should only cost around $100 to have it at your door,lol.

[Funky_Fish14]

You know, it really doesnt matter when its 'ready'. You CAN start using it at any time. Phytoplankton naturally occurs/grows in your aquarium, and even if you are dosing a lower density, it still benefits your aquarium. You could also just add more. You can keep a couple cultures going and restart each one every few weeks, just to make sure you dont end up with a crash, but otherwise, you can really use it at any time. And if you mean for storage, who says you have to store a month's worth? Store a few weeks worth, grow some more for a week or two, and then store more.