I’m finally in the middle of volume seven of the series.
I know that time is all screwed up in the various worlds, but I wonder how much subjective time passes during the entire story.
I don’t think there’s really any way to answer that question. As you read more, you’ll understand.
From first word, first page to last word, last page - one year*. Bev Vincent’s The Road to the Dark Tower covers this, as the seasons are fairly evidently changing throughout the series.
*This discounts any massive amounts of time that seemingly pass after Roland and Walter’s palaver in The Gunslinger, as well as any flashback scenes in any of the books.
That question really can’t be answered without knowing how much time passes after Walter and Roland palaver and without reading all the way through the last book.