How long is a piece of string?
It depends entirely on how long it takes for someone to read your post and then reply if they feel so inclined. It could be a minute... or you may never get a reply.
For example, from the time I noticed your other post, it took around 3 or 4 minutes before I was ready to post a reply. Likewise for this post.
You only get notifications if you check the box accordingly.
