I'm no expert on this but I think there are 4 things to think about:
1. The amount of content you initially create.
2. The amount of original content you create.
3. The length of time you keep the unpublished hub 'simmering' on your account list.
4. The subject/topic you're writing about.
In my experience if I write a hub and keep it simmering for 2/3 days it'll get into the 60's, even 70's before I publish.