i read an article the other day and i think it said the google crawls the main page every 2 days and then the hole site once a month.. not sure if thats true or not ill try to find the article again and edit this post once i do.
A bit gutted, as my homepage has no rank yet one of my subpages is a PR3. I have no idea why this has happened as both pages were optimized pretty much the same at the time.