Logger for org.eclipse.milo.opcua.sdk.client.subscriptions.OpcUaSubscriptionManager on DEBUG would log things when PublishRespones do arrive, though.
The expected keep-alive interval is agreed on when the subscription is created. If the server supports diagnostics you can view the Server diagnostics for that connection in Ignition and drill down into each subscription. Look for the maxKeepAliveCount and publishingInterval parameters:
This means the keep-alive interval is 500 * 120 = 60000 (milliseconds). So a keep-alive every 60s in the absence of data changes. The watchdog timer will fire after 125% of whatever this value is has elapsed.
Ignition tries to create subscriptions with a 60s keep-alive interval by default, but the server is allowed to revise the requested maxKeepAliveCount, so it may be something different.