It is so difficult for me to do this because I am so anxious for them to bloom. How long does cutting back delay the blooms? This is only my second year for phlox so I am not experienced with them.
you do not cut them back as you might a mum ...
or i am totally misreading your question
let them bloom .. then cut off the head ... as the last of the flowers are fading... BEFORE seed matures... and you MIGHT get a second lesser flush.. depending on how early fall comes..
of course.. now that i reread.. you dont mention.. what kind of phlox ...
You can cut them back to prolong bloom if you like. I tried it one year but prefer not to as they bloom much fuller without it. I do deadhead and get a second bloom though.
They can also be cut back for a tiered effect. Deer come through and cut mine. I prefer to wait until after first bloom because find the blooms are smaller when pruned. For me bloom is about two weeks later when pruned.