[Logician]

There is no easy way, because the MYSQL table that saves the subscriptions do not keep subscription date. So by looking the table, you cant understand who subscribed when. You need to make table alterations to first add a date column to that table then in admin/functions.php edit function sendnotification to retrieve subscribers according to this date column.

However it's not clear to me why you want this feature. If I subscribe to a thread, I would be interested in the reply even if it comes X months later. And even if I dont, it's just 1 click to unsubscribe from that thread or all threads at all. Just my 2 cents..