According to a study by Turner et al. , wide area network links have an average of 1.2 to 2.7 days of downtime per year. This translates to roughly two and a half 9’s of reliability .
I was curious how this compared to datacenter links, so I took a look at Gill et. al’s paper  on datacenter network failures at Microsoft. Unfortunately some of the data has been redacted, but I was able to reverse engineer the mean link downtime per year with the help of Aurojit Panda’s svg-to-points converter. The results are interesting: out of all links types, the average downtime was 0.3 days. This translates to roughly three and a half 9’s of reliability, an order of magnitude greater than WAN links.
 Daniel Turner, Kirill Levchenko, Alex C. Snoeren, and Stefan Savage. California Fault Lines: Understanding the Causes and Impact of Network Failures, Table 4. SIGCOMM ‘10.
 Note that this statistic is specifically about hardware failure, not overall network availability.
 Phillipa Gill, Navendu Jain, Nachiappan Nagappan. Understanding Network Failures in Data Centers: Measurement, Analysis, and Implications, Figures 8c & 9c. SIGCOMM ‘11