It also with just browsing depends on which and how many websites at the same time you visit. How much you scroll and switch in between.
I also get around 5h on average. 8-10h is what you get with lowered brightness and kind of a slow working speed or even not scrolling with your fingers at all but with the spacebar. Just check how scrolling sets the cpu aflame. It creates a lot of load and if you slowly scroll while reading you need a lot of power compared to just flipping down a page with the spacebar each time you reach the end.
Imagine the battery life tests Apple do. Switch every 40 seconds to a new web page. Load that page and do nothing for 40 second until the next comes up. Yet most people read shorter info snippets on the web and click much faster through new websites, also we scroll while reading which is by far the most power hungry operation and completely ignored in Apple's battery life tests and automated tests of most review sites.
Also different websites create different load. Youtube HTML5 video creates much less load then the twitch video flash. Twitch.tv is especially terrible on retina screens in full screen. It will suck your battery dry in 3h.
Similar things go for videos all over the web on different sites like Facebook, Hulu, tv networks, ...
A 1080p video is also different than a 480p obviously.
Here are a lot of people that will tell you they get 8h+ but they move through the web in a snails pace most likely. I can get a lot of battery life just coding because that literally needs no power at all (just typwriter power use) but somebody who does creative work in imovie or processes a lot of photos in a decent working speed will get far less.
You can download the Intel power gadget to see imediately what your Intel chipset needs at the moment (when the dGPU is active you won't see its power draw, ergo only with iGPU can you see the full draw). The screen needs about 5W. If the whole thing needs 10W on average you are at 10h battery life. With the chipset at 15W you are at less than half that.
Some programs also read out the battery draw and calculate the full system draw but you don't get the kind of instant feedback.
