It already gives the reason in the SVN version of ULX, so just wait a little while, or if you're brave, use SVN.
By the way, I'm pretty sure it was originally setup to give the reason, I suspect garry or Valve made a change to stop our method from appearing correctly.
EDIT: Oops, I misunderstood your question. If you want to let them know how long they got banned for, why not stick it in the reason? I don't think I'd want to make it automatic since not all server admins would want the user to know how long they were banned for.