One of the trickier parts in any kind of writing is getting timing down. Various forms handle this in various ways, but in novels this is often handled through chapters.
A chapter is the novel’s way of keeping everything from happening at once. It may be made up of scenes that present different moments in time, but if multiple things are happening in different settings at the same time, then they are all presented in a single chapter.
Effectively, if you find yourself saying, “Story thread A. Meanwhile, Story thread B…”, then those moments of each story thread should be woven together in the same chapter.
There are of course exceptions to this rule (a favorite example is Brandon Sanderson’s The Way of Kings, where such organization would drive the reader crazy). But if you’re finding yourself confused by where to end a chapter, using it as a means of organizing the timing of your events is a good place to start.
I’m not doing NaNoWriMo, but if you are, then I’m cheering for you and wish you luck!