To expand on this based on excellent work by @Jeff_AZ . See The Lightning Lane Thread! All WDW Info Here
- For on-site guests, LLs can begin to be purchase at +7 days
- Based on emperical testing, the purchase window will span multiple contiguous split stays for up to 14 days (not the full 16 days of your stay)
- Multiple LL related items for a given day can be purchased / booked the within the same transaction. Each day is handled in a separate transaction.