I started lexapro about a week after my daughter was born. I started feeling it "kick in" after about a week, and by two weeks I was feeling pretty normal again.
I started taking lexapro again when I was about 5 weeks pregnant, and it did take longer to kick in this time (about a month) but I think that's because of all the hormones and changes going on in my body.
I will say, lexapro always makes me feel worse before I feel better. Hang in there. Waiting is always the hardest.