My experience with two 2015 rMBPs and an Air has shown that for light work, 10 hours is a completely reasonable estimate, which matches what Apple estimates.
Of course driving an external monitor, streaming video from the Internet, and using your computer for more than just word processing and web browsing, your system will use more power.
Ditto on the 2015 rMBP. The moment you get beyond light work you're looking at 5 hours but for light internet browsing it really does push towards 10. It's awesome.
Well, my almost an year and a half old Dell Inspiron 5558 gives me 5 to 6 hours (gave 7 to 8 when new) with just web browsing. I don't think that's all that great.
And mind you, I am the kind of person who has managed to accrue about 13 months of hard disk head flying hours according to the SMART stats so the battery isn't exactly in a good condition.
Of course driving an external monitor, streaming video from the Internet, and using your computer for more than just word processing and web browsing, your system will use more power.