Persistency of Seed Nodes A few of the guys at work have asked for me to make them a cryptocurrency (god knows why but I'm happy to oblige). I'm basing it on Cryptonote and have got it all built.

I understand that I need at least two seed nodes to get things going. However, do these seed nodes need to stay on indefinitely or can they be turned off once there's a network of miners?
I'm not familiar with the specific design of CryptoNote, but in general a cryptocurrency node must have a way of discovering its first node in the network. Once it discovers that first node, it can ask that node for all of the nodes it knows of, then ask those nodes for all of the nodes that they know of, etc. etc. until it knows about every node in the network. In most cryptocurrencies, there's no difference between a seed node and a regular node.

There aren't a lot of constraints on how you solve this problem. You could have a seed node at a fixed IP address, and distribute an update if that IP address changes. You could have the node look up a DNS record that corresponds to a handful of nodes. You can have the node request a webpage, and add all of the IP addresses on that webpage to the address list.

Bitcoin uses a combination of fixed IP's, plus DNS seeding. See peer-discovery.