How long is the battery life on an iPad 2?

You will have to explain what you mean by "off and on reading". If the iPad was not sleeping at all for the entire 10 hours, (the iPad never went to sleep at all) you are getting what Apple suggest that you will get with a full charge. Of the iPad was asleep for wven one hour total, you are still not doing bad.

https://www.apple.com/batteries/maximizing-performance/#ios

Your battery is designed to retain up to 80% of its original capacity at 1000 complete charge cycles. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers a battery service for $99, plus $6.95 shipping, subject to local tax.

http://www.apple.com/batteries/service-and-recycling/

Found it:

iPad 2 - Technical Specifications

Apple says 10 hours runtime (not "life" in this context)

Our iPad 2, bought almost exactly four years ago, would demand a trip to the charger at around 10 hours when new, so I would say don't stress about it. Yours is performing fine. At four years I'm not sure mine will do what you are getting but it has been some time since I was on so long a flight. Also, I expect my battery to be "stressed" because the iPad was my wife's before I got it, and she would plug it back into the wall socket as soon as she stopped using it. The battery was not, IMHO, properly exercised while in her custody.

battery runtime -- how long a device can operate before requiring a recharge, usually measured in hours

battery life -- how long a battery functions before it can not longer perform its task, usually measured in years

My iPad Air 2 (only 14 months old) only gets about 9 hours out of a full charge with constant use. I call constant use not letting the iPad sleep for more than a couple of minutes at a time during the day. I use my iPad all day long at work. I wake the iPad at about 7:30 or 7:45 when I arrive, put it on my OtterBox stand, and I'm ready to roll for the day. By 4:30, it is pretty much drained, if not totally drained.

I was getting 10-1/2 hours and more, before iOS 9.2.1. I swear iOS 9 added more time to my daily use. iOS 9.2.1 took care of that.

I have always recharged my iPads every night no matter how much battery power was left and up until iOS 9.2.1, I was getting 10 hours plus. Not the case any more.

I'm with you on iOS9. However, although I did see what I believe to be a dropoff right after getting to 9.2, it seems to be doing better now. My problem with getting an accurate runtime read on my iPad is that I am a computer guy more than a Pad guy and tend to be on the iMac or MBP much more, plus haven't had any long trips in nearly a year. I have a long car trip coming up next month during which I plan to catch up on some ebooks and music while in the car. That will be a better time to check it.

The good news is that a four-year old iPad 2 is otherwise running just as well on 9.2.1 as it did when new. iOS 9 also got rid of some aggravating interface bugs Dear Wife and I both encountered in OS8 on the iPad 2 and her newer iPad Air (bought Dec 2014). Those being gone were worth a little loss of runtime, at least given our usage patterns.

10 hours seems fine if you were using the iPad on and off throughout one charge cycle.