I think it takes a few weeks for SSRI's. I've been on Prozac for years off and on and consistently for the past two years.

My depression returned at the end of 2006 and my p-doc added wellbutrin and I tried it for a month 1/2 it made me nuttier. So, we increased Prozac just yesterday to 50mg by alternating 40mg and 60mg every other day...

Prozac is great for me. Maybe your doctor could try that if Lexapro doesn't work...