15 days is fine. I wouldn't go any longer though.
Depends on resin content as far as humidity goes. Ive grown quite a few plants where 65%+ is just too moist. it doesn't break up or grind right. Get into plants with a roughly 40% resin content and 30%+ thc content and 55% is perfect usually.
Honestly, for my tastes, 72% is pretty much always too moist. Most buds Tend to be tacky and not burn right that high.
You want the stems to snap in the middle, and the flowers, honestly, to feel cracker dry to the touch on their surface at first jarring. Youll still have a small amount of moisture in the stem inside the bud though, and this is what you want to cure out into the rest of the flower, and level down to whatever seems appropriate for the texture and consistency of the flower and its resin content, ime, anything from 55% to 65% can be right. The humidity will also rise in the jars as they sit, and this moisture evens back out into the flowers material.
The other purpose of a long period cure is to let some of the thc-a naturally decarb into delta-9 thc, as this increases potency, and chlorophyl will be largely broken down after about 30 days, and cellulose will burn cooler after a bit longer, giving a better, cooler, more consistent burn.
As long as you never let it get below about 40% once cured up, you can always slowly rais it with those RH packets to whatever seems right for the flower without volatile terps getting much if any damage.
