How long can the bean casings remain in the alcohol when making vanilla extract? Does it matter if they're split, scraped or left un split?

I just leave mine in the bottle. I buy a small bottle of cognac (flask sized) and drop in a vanilla bean. Add more cognac as needed... great stuff!

The bean casings can stay in there for months, and you can even top off the bottle with more alcohol as it gets used up. Definitely split the beans lengthwise, but no need to scrape the seeds as they'll all be exposed.