In my experience (I’ve done this before successfully, as have many of us) at about 3 weeks, I just started spraying the one I wanted to turn into a boy. That way, it started soaking in before actual flower and was ready to go (not sure how ‘scientific’ that is, but in my brain seems logical 
).
Sprayed at least twice a day (there were some days I would miss on/off but not many), until I saw the pollen sacks, and even then, I sprayed for probably 2 weeks into the flower, just to ‘cement’ the transition.
Colloidal silver just straight out of the bottle in a sprayer.
Liquid Silver 16 oz. 240 PPM , Silver MTN Minerals, (Medical Purity Silver Most Bioavailable colloidally Suspended Nano Particles) https://a.co/d/bKjEkI3
You’ll do fine! 
Start spraying, and I’d spray more frequently than 1x a week, personally.
And @ReikoX is correct, start the pollinator plant 3 weeks prior.