I don't have a time time but my 12.9 easily gets me through whole day with TONS, as in 60-70%, left. With 12.9 battery life probably won't/isn't a problem. Can't say how it compares to 9.7 though?
Haven't had my pro long enough to give a in depth analysis, however you would be hard pressed to kill either in one day of heavy use. I watched Netflix for hours on my pro, and it only took away 30% of the battery
I don't own a 12.9, but I'm pretty happy with the battery life of my 9.7. I'm also coming from an old iPad 2, and I'd say the battery life is on the same level as my iPad 2 when it was new in 2011.
Love my 12.9" battery life is literally in the all day span. 10-12 hours usage is what I normally see when using split screen apps and my Apple Pencil. I did notice AirPods seems to be draining them more than anything but that is likely software related.
Over the holiday I used my iPad to watch Netflix and Hulu and didn't think to plug it in until I was going to sleep.