The posting of the notice is required by law to insure that all proper parties know of the foreclosure and have an opportunity to stop it or exercise other options. You can take it down, however, right away. You will also get the same notice by mail and certified mail.
It really depends on the lender. Some lenders will issue the Notice of Sale immediately after the 30 days have passed, others don't do anything for months. I've seen both scenarios. If and when the lender does decide to issue the Notice of Sale, it wille posted to your front door.