To the best of my knowledge, there is no official standard set for WAN reliability, which is why we read so much about service level agreements on product manufacturer websites. I’ve supported service providers who strive to achieve 99.9% network availability and mostly stick to it. The key is in how availability is measured. There are technical measurements, such as pinging interfaces at fixed intervals to calculate uptime, but there’s also an administrative side. Scheduled maintenance and other planned outages have to be added to calculations. Where response times and data integrity go, the waters are muddier. In any case, you have to set your own standards through service level negotiations.