Normally, you can edit the first post within a trade topic for 3 days. As a site customer you can edit the first post for 5 days, as a verified member you can edit the first post for 7 days.
By editing the post during that time the timer resets. So if you remember to edit the post within the 3/5/7 days period you can edit the post for as long as you keep doing this.
You can find info on all customer and verified user perks here:
https://forums.d2jsp.org/verify.php