Most ISP's dns cache servers honor the TTL defined in the authoritative SOA records, unless is 1 minute or less. I think the average TTL time for a dns zone w

Most ISP's dns cache servers honor the TTL defined in the authoritative SOA records, unless is 1 minute or less. I think the average TTL time for a dns zone would be measured in minutes. It needs to be that low in order to do SRV load-balancing, A/B testing, etc. In any case, in the highly unlikely event that they manage to overload the 13 servers, there's plenty of time for every domain to temporarily extend the TTL on March 31.