im of the opinion that a slow burn is less of a specific word count thing and more of a percentage. like a slow burn's resolution, in my opinion, is ideally after the 80%-ish mark so if its like 15k words it would be like 13k if it was like a 80k fic it would be at 50k-70k ish. that being said i do think the optimal word count is such a hard concept to quantify because like each person has their own preference for their preferred length to read. like i think for me the sweet spot is around 10k-40k, but also ive loved 300k fics that were slow born and ive loved 1k fics. all that aside i think as writers we place too much weight on word count in a way most readers dont? like i rarely actively find myself checking the word count for fics when im reading as much like tags yk?
also another factor is like how verbose vs concise someone's writing style is. i think two people can both write a great slow burn and one would be 90k and the other is like 30k.
the hallmark of a successful slowburn is like creating prolonged and intense buildup and tension and pining to the point where you or the reader has to be like omg kiss already. because slow burns are about delayed gratification and the anticipatory factory.
i hope this helps ! i also think having someone else beta read and offer their own perspective is also really helpful in gauging if it feels satisfactory.