Mine is not a typical use case, but I always used my SP4 (i5) with flight mode and battery saver on when taking notes in onenote and could generally see 7-8 hours without a problem. Although that is more hypothetical than tested, just extrapolating based on drainage per hour in lectures (around 10% an hour, but it would vary depending on the fast ring build I was running on a particular day). When travelling to and from london it would be a 3 hour coach ride and I'd watch films and have well over 50% left by the time I arrived.