There are 4 forums here covering 4 generation that spans 5 years each. Wikipedia is not really clear on this (there is no Subaru Outback entry, just Legacy). Anybody knows when the next gen coming? 2015?
It all depends on the market. Subaru has always had its own niche so it can pick and choose when it wants to make a change. Nowadays manufacturers are designing vehicles with more future proofing in mind. You'll see mid cycle refreshes featuring things like you get with the '13 update. Don't always count on them though. If a vehicle is selling well in a weak market it can soldier on for awhile. Take the WRX for example. The Impreza got a redesign but the WRX is still running on previous platform.
In other words, we can't predict when a vehicle will get a redesign but you can keep an eye out on the internet and in periodicals for news and rumors.
The current Outback is selling well enough to where it could remain in its current form until 2016.