I am new to flasking my own stuff and i was woundering after filling the flask with medium how long does the medium in the flask stay good if i didn't use them all the first time i flask.
Prepared flasks should last until they either dry up or contaminate. I'm sure the life of media is not indefinite but they should store for at least a year no problem.
It also gives you a chance to weed out the flasks that contaminate as part of the cooking process. And you know that any contams you get from there on in, are obviously down to the sowing or replate process.