Magento has announced that Magento 1 will be supported for 3 more years after Magento 2 has been released (is out of beta). The "General Availability" will be end of 2015. So Magento 1 will be supported at least until end of 2018!
If you look at other software, Typo3 for instance, you will see that after those 3 years you could probably buy more years of support from Magento itself. Magento won't support Magento 1 longer, as it is no longer profitable, but if there are many Magento 1 customers willing to pay, then it will be profitable for Magento.
In a lot of answers I read that people think there will be a lot of (or some major) parties that would be giving support on Magento 1 after those 3 years. But I don't think this support is that reliable as it is splintered over so much parties and could end up with compatibility issues between those split offs and extensions from vendors.
By the way, if you look at Magento's Imagine event as being the place for big announcements, I think Magento 2's official and festive release will be April 11-13 at Magento Imagine 2016. So Magento 1 would then be supported until April 2019 ;)