Microsoft today made a major announcement regarding Windows Phone 8 support lifecycle in order to push the devices to enterprise users.
Microsoft is increasing the length of the support lifecycle from 18 months to 36 months. So, beginning with Windows Phone 8 Microsoft will make updates, including security updates, available for 36 months. But usual conditions about operator support, etc, apply.
These updates will be incremental, with each update built on the update that preceded it. The mobile operator or phone manufacturer may control the distribution of these incremental updates and update availability may also vary by country, region, and device hardware capabilities.
Source: Windows Team Blog