I glue a paper label to every new spool as soon as it comes out of the bag, and record the weight. Then I dry the spool in my X1C (no AMS) until it stops losing weight. Every spool has lost at least two grams in this first drying, some have lost as much as 8 grams.
I record the date and weight on the paper label, marking the weight again after use, before placing it back into the bag with desiccant. I weigh it again when it comes out of the bag. If it has gained more than a gram or two the spool (and the desiccant) is dried again.
My printing is intermittent, so I do not usually leave a spool on the printer. The most hygroscopic filaments I’ve used (TPU, PC) have not gained enough moisture during use to be a problem, but those prints only took 2-3 hours.
Average daytime humidity here is 60%, nights are over 80%, 95%+ is common. I have left PETG out for a few days at a time without problems, but I dry it again before storage, usually losing a gram or two from a partial spool.
I don’t have any faith in the inexpensive hygrometers or color cards available. I do not think they are either precise or accurate. At best, they show the moisture content of the air, not the filament. It’s easy to find pictures of several in the same environment but displaying wildly different amounts of humidity. I will probably get a dedicated single spool dryer box soon, but my plan is to set it on my digital scale.