Depending on what you are aiming to accomplish...
If the aim was to prevent relatively new members from starting new topics while allowing them to post replies -- you could accomplish that by setting up a separate user group for newbies and give them restricted forums permissions for 'creating new topics'. Once they reached >x number of reply-posts, they could be bumped into a regular user group.
Although not the time limit you were looking for, I thought it might be semi-useful concept in considering options.