Common sense (at least to me) suggests continuous light (which was how the 56 hour figure was given) and usage for 30 minutes over an extended period of time are two very different things. 56 hours as measured by Kobo won't equal 4 months usage as measured by Amazon. I'd bet they both have very similar battery life.
Edit: In fact, from the
Kobo Glo experience page, it states - "a single charge on the Kobo Glo lasts up to a month so you can read as long as you want"
Considering that's what the Kindle used to say before they changed from the 1 hour per day to 30 mins per day (to match up with the Nook), I'd say that pretty much confirms it.