Increasing the Lifetime element for EWS Streaming Subscription Connection

Using Microsoft's EWS, we're able to listen to a mailbox and take actions when a new email comes in. However, I can't figure out how to avoid the connection timing out.

Per Microsoft, here is the constructor for a StreamingSubscriptionConnection:

public StreamingSubscriptionConnection (
    ExchangeService service,
    int lifetime
)

In my app, I've coded it as follows:

service = new ExchangeService(ExchangeVersion.Exchange2010_SP1);
StreamingSubscriptionConnection conn = new StreamingSubscriptionConnection(service, 30);

In other words, I've got the timeout (lifetime) set to 30 minutes, because that's the highest I've been able to set it. How can I increase this? Or, how can I trick this subscription into staying alive, even if ~45 minutes transpire between incoming emails?