I don't. I try to stay within the norms of the average novel of my genre, but beyond that, it's however long as it needs to be.
For me, it's just experience. My big blobs of story finish up, after revision, at between 110K and 135K - this has been consistent over numerous books.
Whew. Read a few" how to" writing books which mention 90K normal limit for crime novels. Before putting any words to my first novel. After my first draft i'm at 131K. I got there because I felt each scene i wrote should result from the previous one and hint at the next scene.. Realize I'm corresponding with a Veteran so I'm sure you're well aware of what I'm saying. Just wanted to hear from someone experienced in writing that it's ok to zoom past a 90K limit.