Initially when SharePoint 2013 was released the stretched farms were unsupported. After continuous efforts to review and update the product performance and capacity boundaries, stretched farms for SharePoint 2013 are supported as of April 2013. For a stretched farm architecture to work as a supported high-availability solution, the following prerequisites must be met:
- There is a highly consistent intra-farm latency of <1ms, 99.9% of the time over a period of ten minutes. (Intra-farm latency is commonly defined as the latency between the front-end web servers and the database servers.)
- The bandwidth speed must be at least 1 gigabit per second.
Here is the official statement from Microsoft: http://technet.microsoft.com/en-us/library/cc262485(v=office.15).aspx#hwLocServers