On creation of toast, set date-created attribute to Date.now() and then have a recursive timeout to update the subtitle. On notification dismiss, destroy the chain.
Dirty non-working example:
function updateToastStatus(toast) {
var data = toast.data();
startTime = data.created
// logic to update subtitle to show now, 5 seconds ago, 10 min ago, etc
setTimeout(updateToastStatus, 5000);
}
updateToastStatus(toast); // First call starts process

Planning on taking care of this at some point, I'll update here if I do.