I'm afraid I don't fully understand your question. Perhaps you can clarify?
If you mean how long does it take for an old, inactive thread to be auto-locked, it varies for each forum. Between 3-6 months, usually.
If you mean how long it takes a thread to get bumped off the first page, again it varies for each forum and how active each forum is.
If you mean how to double-post to "bump up" a thread, one can do that every 24 hours, minimum.