When using the pubsub java client, how long should a Publisher object live for ?

In a scenario where we get frequent HTTP requests that get translated into Pubsub messages, doing

try {
    publisher = Publisher.defaultBuilder(topic).build();

    //... do publish stuff
}
finally { 
    if (publisher != null) {
        publisher.shutdown();
    }
}

each time seems inefficient. I couldn't find any documentation on the expected lifetime of the object.

How often should the publisher be shutdown (either in terms of time or messages published) ?