Hmmmmmmmmm this is hard to answer for me, because the composition of a fic and what folks take from them is always different for everybody–for some, a lot of time might seem extraneous, while for others, it’s delicious.
In my opinion, it’s not the length of the fic that matters the most in a slow burn, it’s the progression of the relationship. It depends on where you start. If you start with hate, it’s going to take you a long time to get to love in a believable way. It also depends on what’s actually burning. Is it the sex? The emotional connection? These two things can definitely happen on a different timeline.
I’ve been ready to climb the walls with some slow burns I’ve read that I’ve lived on for even a hint of something between the characters.
In my experience, the longer I had to wait with an engaging story, the more ABSOLUTELY FREAKING LIBERATING it was when they finally realized how they felt about each other.
Gravity Well, for example, is the first slow burn I’ve written, and I actually intended things to be shorter…for culminations of feelings to occur within the next 10k. However, at 72k, I’m not even close to getting my main character worked around from where I started to where I know he’s going to end up. What I thought would be a 80k fic will probably be more like 140k. I hope it’s worth the wait, in the end.
Ultimately, I’d say if you’re working on writing a slow burn, don’t put chapters / words in because you’re worried it needs to be a certain length to “qualify.” Just write the story from beginning to end the way you think it should happen and the way that feels believable to you!