I'm a heavy user. I use mine aprox 3 hours a day together with the wireless Media Adapter, BT always on, Wif always on. Including streaming.
With my tips, it is possible to get > 7.5 hours with heavy use.
The only thing you have to look for is processes which are not necessary at the time. It's called resource management in IT, which is absolutely applicable on the SP3, when you use a little bit of common sense
The only thing which can kill your batteryusage is bad wifi access, which unfortunately in Holland is mostly the problem on Universities etc
See I wouldn't describe you a heavy user though... On my busy days, I'm in University for 3 classes, a lab, and a tutorial spread out over 8 hours. WiFi on the entire time, One Note and Word open in every class, along with multiple PDFs, Powerpoint, BT, etc. Avoid video at all costs during this time, as well as any browsing that's not related to schoolwork, and I'm lucky if I got 5 hours.
During the 4 months I had an SP3, I was on it around 12-15 hours a day between classes, HW, and leisure time after class. At school, the majority of its usage was on super-dim brightness, like 10-15%, while at home it was always at 80%. I averaged just under 5 hours with this setup according to powercfg /batteryreport. I then purchased the power cover and got around 7 hours, which sometimes let me get through a day at Uni if I managed to find a plug outlet at the cafeteria during lunch.
See how the definition of power user varies?
For some comparisons to think about, the problem with the SP3, is that MS advertises it as the tablet that can replace your laptop, while specifically attacking the MacBook Air, which gets a legit 10-12 hours of all-day usage, when the SP3 is advertised as getting 8... which just isn't the case under the same sort of power usage. The 13' MBA has a 54 mWh battery, with a far crappier resolution, and dimmer brightness settings than the SP3, while the SP3 has a 42 mWh battery...