How long do you give meds to work for OCD?

I have been dealing with OCD for a very long time.  I was diagnosed over 15 years ago when it was a new diagnosis.   The only med I've found to work for me is namebrand Prozac at about 40 to 60mg.  However, after being on this for a few years it seems to not be as effective.  I've gone thru this before.   This time I am trying Pristiq.  I have been on it for about 7 or 8 weeks.  I don't feel any better.  It is probably helping some.  It's not like when I go completely without meds... but it's not helping me to where my life is enjoyable yet.   Now my medical doctor prescribes me my meds instead of a phschiatrist due to the fact that my OCD is controllable as long as it's being treated.  I've been thru all the threapies and several different meds.  Plus I am not suicidal.  Mine is simply an issue of getting my meds right.  Anyway... Should I keep going with Pristiq for another month?  Or try something else?  I remember when going to see a pschiatrist.. they would ask me every 2 or 3 weeks if I was feeling any better.    This time .. after I had been on Pristiq for about 2 or 3 weeks I thought it was getting better.  But now.. it seems I have fallen backwards.  Any ideas?    

Meds I've tried that didn't work for me include:  Lexapro, Zoloft

Have any of you had much success with Luvox?  I am thinking about trying that next time.  

PLEASE NOTE:  I realize that different meds work different for different people.  So just because something may not be effective on me.. doesn't mean it won't be effective on someone else.