milk spray

how far into flower can i apply milk/water mix to prevent powder mildew. thanks

I spray whenever I like in flower, as long as you're not using chemical sprays it should be good...which, judging by your signature you're not.
Spraying in flower can be a problem because it can help rot/mildew by creating a damp habitat, and we don't want to smoke chemicals.
This all seems so simple I have a feeling I've forgotten something...

Get or keep good airflow?

dont spray anything, to keep from getting powdery mold just make sure theres not too much moisture, if your indoors get a dehumidifier if humidity is a problem and make sure air flow is good, or get a strain that can take moisture and train your plant to grow spread out buds like a scrog so if bud rot does occur theres plenty of other buds that arent effected and that one can just be cut off.