Seibertron wrote:after X amount of time, the topic reply notifications stop
No. You're notified once per topic. Then you have to re-visit the topic in order to get one (and only one) notification upon a reply. This concept makes sense because it reduces the amount of sent e-mails to a minimum (the amount of e-mails sent from a host might also be a factor for others to classify you as spammer) and also hardly no e-mail is wasted for users which aren't visiting your board anymore.
There is no setting, it's a concept and you'd have to modify it.