P2Pool should immediately call getblocktemplate after it knows that there is a new block. It is notified that there is a new block via its P2P connection to bitcoind, through which bitcoind sents an inv message when it has a new block. The code that does this is the work_poller in node.py. The reason that you saw a 18 second delay is something within that loop. Maybe bitcoind was lagging and took a long time to announce the block after it printed "new best"? Or maybe P2Pool was lagging and took a long time to respond. Is the delay always long or just for this one block?

That said, P2Pool will generate its own work if it knows that there is a new block (via its P2Pool peers) and bitcoind hasn't given it new work yet. These blocks will be empty, though. It's just a stopgap solution for bitcoind sometimes taking a long time to catch up.

In other news, a lot of users (or one big user) upgraded to v15 in the last day, and we'll probably be fine when BIP65 takes effect.