Hi Elizabeth,
First of all, I have never had much luck with topical anesthetics, no matter how I apply them. That said, I can tell you that the cream should be on at least an hour beforehand.
The manufacturer's product insert for prescription EMLA (2.5% lidocaine and 2.5% prilocaine), says a "thick layer" of cream should be applied to intact skin and covered with an occlusive dressing for at least one hour. An "occlusive dressing" can be Tegaderm dressing or plain old plastic wrap from the kitchen. "Thick layer" means 2.5 grams of cream for 20-25 cm2 of skin. For "major dermal procedures", they recommend 2 grams of cream per 10 cm2 of skin, allowing it to stay in contact with the skin for "at least two hours".
"Dermal analgesia can be expected to increase for up to 3 hours under occlusive dressing and persist for 1 to 2 hours after removal of the cream." (Yeah, right. I think 30 minutes is closer to the mark.)
"Occlusion is not necessary for absorption, but may be helpful to keep the cream in place."
Hope that helps.
~Terri