CT + symptom improvement after VP shunt

Hi

My 83-year old father has NPH.  VP shunt was implanted in Novenmber, 2010.  However, his symptoms (urinary incontinence, excessive daytime sleepiness) were still present.  CT was done and did not show any improvement from the one before surgery.  So the pressure was turned down from 120  to 90.  I've 2 questions:

1.  How long does it take to see improvement in his symptoms ?

2.  How long does it take to see improvement in CT ?  

Thanks for any info you can share.  Andy