I harvest the seeds as soon as the pods open allowing the seeds to escape. Look at it this way, if ma nature is letting them go, they are probably ready. It's not like fruit where you let it ripen in a bag, it's when the seed is viable and ma nature says - ok take them.

If your main goal is to keep your plant blooming - keep up on deadheading. That is - instead of letting seed form - snip off the whole stem of all spent flowers - down to the next set of healthy leaves. I do this about every other day all summer...and I keep many things blooming 12 month of the year...when the non-dead-headers only get a few weeks. I also get a LOT of seed along the way too.

I often put the well-formed pods in water to continue ripening seed. My sunroom is jammed with these kind of cuttings! If you let the plant mature the seed - it thinks it's job is done and it dies. You can let a few pods mature on annual plants at the end of their season - or when it looks like weather will kill it anyway.

For perennial Poppies - you may want to let some large ornamental pods form, but plant some other kind of later maturing foliage or bloom nearby, and cut back dying folaige to keep the area looking neat, as the plant will be a goner for this season.