i don't believe it's going on too long at all, after all red john is
the big bad, if they caught him after a series or 2 then that would pretty much be the end of the show, or they'd have to find a really awkward way of dragging it on.
also this should've really been posted in the mentalist thread that already existed here
https://forum.rampant.tv/showthread.php?tid=2111 instead of creating a new thread.