Hope you're feeling better.
Everybody is different. They usually take 4-6 weeks to be effective, if at the correct doseage. With some of the SSRI's and SNRI's, the side effects are increased anxiety, agitation, etc. Give it a few weeks if the symptoms are bearable. If not, don't hesitate to ask the Doc for something else.
Are you taking any benzos in the meantime as they will give some immediate relief until the Lexapro kicks in?